D-DART: A Double Deck EMU With Three-Axle Bogies

The author of this article also decided to create a version of the D-DART suitable for use on the rail network in continental Europe. He developed this without reference to any particular passenger rolling stock manufacturer, and so far this concept remains at the feasibility study phase.

What The D-DART Concept Involves

The one major advantage of the D-DART design, an articulated rake of double deck passenger stock sharing three-axle bogies, is that it results in a higher passenger capacity per vehicle, this becoming more important with increasing patronage of many urban rail networks, caused by rising urban populations, and a shift from private to public transport. We take as our examples two capital cities - London and Praha.

The D-DART is based on two interesting design concepts for complete rakes of rolling stock which nowadays have almost been completely forgotten. In East Germany Deutsche Reichsbahn developed shared three-axle bogies for its Class DBv double-deck push-pull rakes. However it was discovered that the force exerted on the rails by the three-axle bogies using a virtually rigid wheelset guiding system was too great, resulting in an excessive rate of wheel and rail contact wear, and a higher risk of derailment.

The second design concept is rather more recent, and involves moving the bogie pivots out of their central position. This concept was introduced by British Rail on the APT EMUs (designated Class 370), and has since been incorporated in Stadler’s GTW DMUs and EMUs and on articulated trams mounted on bogies which have a limited pivoting range. The Class 370 prototypes suffered from production delays, the decision by BR to simultaneously develop a large fleet of diesel-powered Class 43 HSTs, and various technical problems. The APT research team was disbanded in 1980, on account of the lack of government funding.

Nevertheless, neither the three-axle bogies in the „Doppelstock-Gliederzug“, nor the modified bogie pivot design in the Class 370, can be regarded as failures. The use of three-axle bogies and articulation meant that longer - and heavier - bodyshells could be created than in the double deck stock on two-axle bogies. The off-centre positioning of bogie pivots results in improved negotiation of curves, improved kinematics, and enables the provision of a double deck for practically the entire length of a bodyshell.

Design proposal for the British version of the D-DART EMU.

The design of the D-DART involves each of the carriages at either end of the rake of stock being mounted on two-axle bogies at their outer ends. At their inner ends they share three-axle bogies with the adjacent carriage. The two outer two-axle bogies are non-powered. Those three-axle bogies which are powered have their outermost axles powered. To ensure an optimal weight distribution, it is envisaged that most traction and auxiliary equipment is situated near the end bogies. This will also make the carriages more stable, for instance, when strong sidewinds are encountered.

The bogies and underframes were subjected to a simulation programme so that the mechanical assemblies could be subjected to dynamic analyses. This was necessary to ensure that the design is feasible, and indicated that the running characteristics of the stock meets safety requirements. The simulations also involved a comparison of the ride characteristics of a rake of stock formed of conventional carriages mounted on two-axle bogies, with no radial wheelset guiding. This demonstrated that the proposed underframe configuration, using three-axle bogies in which the two outer wheelsets are guided radially in curves, significantly reduces the force of wheel-on-rail contact compared with two-axle bogies. This means that wheel-on-rail wear is reduced, to the benefit of both wheel and rail.

The first APT gas turbine prototype, APT-E (Experimental), was formally handed over to BR’s Research Division on 8 July 1970. It first took to the rails in autumn 1971 on the recently inaugurated Old Dalby test track. This photo, taken 1972, shows the train at the Railway Technical Centre in Derby. Its maximum design speed was 249 km/h (155 mph). Some innovative features were incorporated, including active tilting on curves, articulation and bogies with their pivot pins located off-centre, towards the outer ends, but on the centreline. When the test period was completed, in June 1976 the four-car train was sent to the National Railway Museum in York for preservation.
British Rail’s Research Division then developed the APT-P (Prototype), a 25 kV AC version, of which three Class 370 trains were built between 1978 and 1980. These had two non-powered end cars with driver’s cabs and passenger accommodation, and two non-gangwayed power cars (each rated at 3,000 kW) in the centre of the formation -in all 14 cars, of which 12 had passenger accommodation.

The London Suburban Rail Network

There are two characteristics which are peculiar to the British railway network:

• platforms are mostly of a standard height of three feet (914 mm) above rail top. Therefore there is no requirement for „low floor“ vehicles to facilitate level boarding.
• most of the British rail network has a loading gauge which is more restricted than that existing in continental Europe, thus largely limiting the use of the double-deck vehicles.

The high platform characteristic has resulted in trains being easier to board by handicapped travellers, with most modern stock being practically barrier-free in this respect. However, the loading gauge restrictions have in recent years become considerably more irksome, both as far as the design of both passenger stock and also the transport of High Cube containers are concerned. Simultaneously, there has, over the past couple of decades, been a phenomenal increase in patronage of passenger services, in particular within the extensive London commuting area. The British Government has been obliged to adopt a pro-rail policy, and to investigate ways in which the rail network can accommodate more passengers.

Two strategies can, in theory, be adopted, to increase railway capacity:

• increase infrastructure capacity through signalling and civil engineering upgrades, to enable more frequent or longer trains to be operated,
• increase train capacity. This involves either longer trains, or the use of double deck trains.

Both strategies are, to an extent, mutually independent. To introduce longer trains, one has to ensure that there is adequate platform length at stations. To provide a more frequent service, it will often be necessary to upgrade train protection systems to enable shorter headways. To provide double deck trains, loading gauge clearances must be adequate. In all instances, infrastructure investment, as well as investment in new trains, is usually involved.

One railway which exhibits potential for the use of double deck stock is the Great Western Main Line (GWML), originating at London Paddington and serving Bristol together with a large arena west to London. It is the electrification of the GWML, and associated infrastructure works, as outlined in Network Rail’s plans for Control Period 6 (2019 to 2024), which makes this route of particular interest nowadays for the possible operation of double deck passenger trains. Some of the investment in loading gauge enhancement can be recouped by the advantages gained (through revenue) in operating high capacity double deck trains.

The data shown compare the weights and passenger capacities of these four types of EMU, which are all of identical length. (click to enlarge)

Those who have read Railvolution 5/16, will recall that this is not Britain’s first venture into the field of using double deck stock. The EMUs developed by Bulleid in the late 1940s were conventional vehicles, with non-articulated bodyshells mounted on two two-axle bogies, designed in conformity with the loading gauge, thus offering only a limited double-deck area.

Efforts are now being made to address the design limitations, and to create a true double deck design suitable for Network Rail loading gauges. One example is the AeroLiner 3000, a mock-up of which was exhibited at InnoTrans 2016. The Tata Steel Halcrow Joint Venture, which requested this study, also decided to take into account the design of other types of double deck stock. It finally selected the D-DART as the most appropriate design, on account of the highly effective utilisation of the areas available for passenger accommodation. The upper image explains the reasons why the D-DART was chosen.

The D-DART makes use of the APT concept of articulated bodyshells and bogies with pivots located off-centre - towards the outer end of the bogie and bodyshell. Analysis of the optimum basic dimensions of the D-DART train relative to the loading gauge (and in particular the platforms) determined the distance between bogie pivots (17,300 mm) and the distance between bogie centers (23,300 mm).

An image showing what the proposed D-DART three-axle bogie would look like. The pair of brown connecting rods are the off-centre pivot pins, at both leading and trailing ends of the bogie. This is a powered bogie with the axle-mounted gearbox input transmission shaft on the left. The traction motor is suspended from the underframe of the bodyshell.

The bodyshell design is a compromise between the need to take into account both the existing constraints with respect to side overthrows (height overthrows were not taken into account because of the above-mentioned reasons), and adequate travel comfort for passengers. The bodyshell, above platform height, is 2,800 mm wide, but limited to a width of 2,600 mm below platform height. The result is that at floor level the interior bodyshell is of sufficient width, the only design constraint being the absence of space under seat unit armrests situated adjacent to windows.

Unlike on most European passenger rolling stock, the bodyshells are straight-sided, and the sidewalls do not curve inwards as they approach the underframe. The lower image shows cross-sections of three proposals for double deck carriages conforming with the Network rail loading gauge.

Bodyshell cross-sections of three proposals for double deck trains for Network Rail suburban lines. On the left is the Aeroliner 3000, which is 3,922 mm high above rail top, and two D-DART versions, offering different seating configurations: the centre version is 4,275 mm high above rail top, the right-hand one 4,500 mm high.

The D-DART is a five-car EMU designed for 25 kV AC 50 Hz operation, 122.3 m long, the train has a maximum axle-load of 21 t. It is mounted on two end non-powered two-axle bogies and four three-axle intermediate powered bogies. Eight of the 16 wheelsets are powered, this ensuring a good adhesion to total tare weight ratio. Each traction motor will have a continuous rating of 500 kW and a one-hour rating of 750 kW. This will give the train a continuous power rating of 4,000 kW and a one-hour rating of 6,000 kW, enabling it to achieve a required acceleration rate of at least 0.77 m/s2.

Compared with other types of suburban EMU, these acceleration rates appear rather modest. However the project promoter envisages the train being used on outer suburban services, of the type operated between London and East Kent by Hitachi’s Class 395 EMUs on HS1. We are looking here at limited-stop services, on which the D-DART will regularly reach its top speed of 200 km/h (the Class 395s have a top speed of 225 km/h for use on HS1).

The traction equipment will consist of two independent circuits, comprising a traction transformer and a four-quadrant converter, these being situated at each end of the train, the converters being located directly behind the driver’s cabs and the transformers under the entrance vestibule floors. The converters will feed the DC link which then supplies each traction motor’s individual traction inverter, the latter being located near the bogies, under the stairs to the upper deck.

The end cars are also used for housing other auxiliary devices, including the auxiliary drive converter, batteries, battery chargers, compressors, main air reservoirs and brake equipment cabinets, sanders, and ATP equipment. All these are situated towards the outer ends of the end cars, near the two-axle bogie.

In the end cars, between the driver’s cabs and the first pair of entrance doors for passengers, multi-purpose areas are to be provided, with wheelchair harness points and a wheelchair-accessible WC cubicle. Two ordinary seats (not tip-ups) will also be provided here for use by helpers. When not being used by wheelchairs, this area can also be used to accommodate up to two prams or six bikes. Six tip-up seats are also provided. Above this particular entrance vestibule the air conditioning units for the passenger accommodation are situated. Each end car has two entrance vestibules, and between these there are two decks, each over 12 m in length, with a total of 115 seats, mostly in 2 + 2 rows, with a distance between seat backs of 800 mm. On both upper and lower decks there will also be four bays, each with four seats, where the distance between seat backs is 1,750 mm.

Although all three intermediate cars will essentially be identical in design, the centre car will not have a WC cubicle. Each will have two decks, each almost 16 m long, and will be fitted with 142 seats (with WC cubicle) and 146 seats (without WC cubicle), in open saloons. The entrance vestibules will be situated adjacent to the inter-car gangways, thus maximising the length of both decks.

The result will be a train which, given its length, offers a significant increase in seating capacity compared with existing single deck trains on the GWML. It should also be noted that the seat to tare weight ratio is lower than that found on other designs of double deck rolling stock. The D-DART’s technical specifications are summarised in the table below. These data were also used in a study evaluating the possibilities for increasing passenger transport capacity on the GWML. This concluded that it would be possible to:

• increase the number of trains in each direction from 16 to 20 per hour, thus offering a 25 % increase in overall seating capacity,
• increase overall seating capacity by 42 % by introducing a fleet of D-DART EMUs,
• by increasing service frequency to 20 trains per hour, and by introducing a fleet of D-DART EMUs, increase overall seating capacity by 78 %.

Patronage on the GWML line is predicted to increase significantly over the coming 25 years. The first of the above options would only provide sufficient capacity until 2027, the second until 2034 and the third until 2043. It has to be borne in mind that forecasting patronage levels for a quarter of a century in advance is an uncertain matter. Nevertheless, if the only action taken was increasing service frequency with a fleet of single deck EMUs to meet demand forecasts for 2027, it would be very difficult to then realise the project of a fleet of D-DART EMUs. Project preparation for these double deck trains must be taken into account.

Suburban Rail Services Radiating From Praha

Many other large conurbations, throughout Europe, are facing similar problems to those being encountered on the London rail network. One example of these is Praha. Here, like in London, many suburban trains share the same routes as long distance trains. In Praha, overcrowding is now being experienced on services operated by three-car double deck Class 471 City Elefant EMUs, in spite of their 620-seat capacity when running in multiple. The problem is worst during the morning peak, on some trains on S1 (Praha - Kolín), S7 (Praha - Beroun) and S9 (Lysá nad Labem - Praha - Benešov u Prahy) services. On S9 patronage doubled between 2006 and 2016, and other routes are also experiencing considerable traffic increases.

This graph shows that alighting times at stations from intermediate cars of Class 471 (double deck) and 440 (single deck) EMUs are very similar, the single-deck RegioPanter having only the slightest of advantages.

At present peak-hour services operate at headways of either ten or 15 minutes. This means that they occupy many train paths, leaving few vacant paths for long distance passenger and freight services, so the question of providing increased capacity needs urgent addressing.

Unlike on Network Rail in Britain, the SŽDC network has sufficient loading gauge clearances for double deck trains. The integrated public transport authority for the Praha metropolitan region, ROPID (Regionální Organizátor Pražské Integrované Dopravy), has announced intentions of acquiring seven single deck 185 m long EMUs with around 600 seats for suburban services. It is stated that these trains will be able to offer shorter journey times, and have a lower price per seat ratio.

ROPID’s justifications for these new trains do not have any solid foundations. It is assumed that single deck EMUs have more lively acceleration than Class 471 EMUs, but this only applies to Class 440 RegioPanter EMUs and up to 100 km/h, as even a double deck EMU could be designed with greater acceleration performance than a Class 471.

As regards the price per seat ratio, the only possible local comparisons are again the Class 471 and 440 EMUs. This is not really an exact comparison because of the historical circumstances of the purchases. If we were to examine the cost of double deck EMUs more generally, it would become clear that the latter type of train is cheaper, either on the basis of a price per seat ratio, or on the operating cost per seat ratio, than a single deck train.

It could of course also be argued that dwell time at stations is shorter for single deck, rather than double deck trains. The author decided to survey the time taken for passengers to alight from the intermediate (Class 071) car of a Class 471 EMU and an intermediate (Class 642) car of a Class 440 EMU. Alighting times from the Class 442 car were slightly, but not significantly shorter - see the graph above. Station dwell time is determined not only by the number of passengers that can pass through the entrance doors in a given time, but also by the configuration and size of the entrance vestibules.

Quite apart from train design and new trains, the Praha suburban network has one specific infrastructure deficiency - many stations only have 200 m long platforms. And these are not only older stations. Most of the new, or rebuilt stations have platforms of this length. At all stations there is a board or other form of sign indicating where the front end of trains must be brought to a halt. This effectively precludes the use of passenger trains over 200 m long, unless the platforms are lengthened.

A three-car Class 471 is 79.2 m long, two in multiple 158.4 m long. Adding another 26.4 m long intermediate car to each train would result in an eight-car formation which is over 211 m long. The maximum possible length for a second intermediate car would therefore have to be under 21 m, to obtain an eight-car formation under 200 m long.

Moreover, there are plans to convert those parts of the SŽDC network currently electrified at 3 kV DC to 25 kV AC 50 Hz. The Class 471 EMUs are at present between four and 20 years old, and would have to be adapted for 25 kV AC operation. During this complicated procedure it may be possible to create either seven-car fixed-formation Class 471s, or four- and three-car versions, which can run in multiple. This could be problematic, both from the point of view of the different traction equipment found in different 471s, and because of the loss of a standardised fleet of three-car trains.

A design proposal for a Praha suburban network D-DART. (click to enlarge)

SBB has found a way round this capacity problem on the S-Bahn Zürich network. Here several types of double deck, four-car, 100 m long EMUs are in service: Classes Re 450, RABe 514 and RABe 511 (some of the latter KISSes are six-car formations). In other parts of Europe there are standard four-car EMUs in service, and these are over 200 m long when running in multiple. The same problem as would be found with the 4-car Class 471 EMUs, if short platforms are encountered.

The D-DART concept would avoid this problem. Such an EMU, in a four-car configuration, is just 98 m long, and would thus, in an eight-car multiple formation fit comfortably within a 200 m long platform length. Each car would fit to a TSI LOC & PAS requirement for a maximum distance of 20 m between the wheelsets of the bogies at either end. The loading gauge would conform to the EN 15 273 standard, and the bodyshells would be 2,800 m wide.

It is thus concluded, following the analysis of current double deck EMUs available on the market, and recent tenders invited by European operators, that the D-DART, adapted to the specifications required for operation in continental Europe, would be a suitable option for future double deck EMUs for the Praha suburban network. The trains would, of course, incorporate many of the features and design principles found on the British D-DARTs. The most significant design differences would be in the curved cross-sectional shape of the bodyshell, and the entrance door and window positions. A platform height above rail top of 550 mm requires the entrance vestibules to be situated between the two bogies.

The table below shows how a family of EMUs could be created on the basis of the D-DART. The trains need not be entirely double deck. For instance, the Combi-DART (C-DART) has single deck end cars, with roof-mounted traction equipment, similar to what is found on Desiro HC EMUs . This could have certain operational and design advantages. The graphs below indicate how the C-DART or D-DART compare with other existing EMUs currently on the market.

How the C-DART and D-DART proposals compare with existing types of single and double deck EMUs.

Conclusions

Technologically, the design concepts incorporated in the D-DART are not new. They were tried and tested nearly half a century ago, but were then ahead of their time and the technologies then available. The three-axle bogie, shared underneath two bodyshells, and the use of off-centre bogie pivot pins, located closer to the leading or trailing ends of the bogie, both favour the creation of longer bodyshells, enabling an increase in seating capacity.

The result is either an EMU or a rake of push-pull double deck stock which, while being shorter and lighter than one of conventional design with two-axle bogies, has a higher seating capacity. The increase in various parameters may be considered marginal, but it is nevertheless an increase, and brings with it various other positive benefits. These benefits could be significant for an operator, or transport authority considering the type of train to be bought for the future, and could interest an existing rolling stock manufacturer to consider developing a range of D-DART trains to offer to clients.

The GWML study for Control Period 6 has now been submitted to the British Ministry of Transport for consideration. Should it be decided to go ahead with the proposals outlined in this study, it is highly probable that D-DART trains would be considered as contenders for acquisition. The result would then be the first practical use of the design, thus enabling it to demonstrate in practice its advantages.