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The bilevel car (the North American English term) or double-decker (other forms of English) is a design to solve the problem of increasing passenger capacity on railway carriages, without squeezing more (smaller) seats into the same space and/or decreasing the pitch (distance between seats).
If one couldn't add cars or increase the length of the car, the only possible direction for expansion is up. This is usually done by reducing the lower floor height closer to the rails and adding a separate upper level floor.
Because of the standard height of tunnels and overhead power wires, many double-deck trains set the bottom deck lower down, between the trucks (bogies in UK and Australasian parlance). Usually the entrance doors are above the bogies, and there is only one deck at the entance area. From there one can go upstairs or downstairs. For example, for the DD-IRM (see below) it is one step up from the station platform to the entrance, and from there seven steps up or four steps down. Some double-decker trains have their entrance doors on the lower level and no steps are needed to get to the lower deck.
Double-deck trains often have curved windows upstairs. In the dark and in tunnels this causes a distorting mirror effect.
Double-deck cars
France
The Chemin de Fer de l'État in France ran double-deck suburban coaches from 1933 and its successor, the SNCF, ran replacement cars, the VB2N from 1975 as well as double-deck RER trains. SNCF runs double-deck TGV cars on heavily loaded high-speed services. Many suburban, regional and high-speed services are operated by double-deck DMUs, EMU, coaches and TGV. The French loading gauge dictates that the double-deck cars have a maximum height of 4200 mm or 13'-9.35".
Canada and USA
Other designs, including rolling stock made by Colorado Railcar Manufacturing, Budd, Pullman-Standard, Bombardier (Toronto (GO Transit)) and others, have the entrance on the lower deck rather than an intermediate level. Amtrak Superliners are double-decker cars of this variety, with the entrance a step or so up from the lowest station platform level, or at the level of slightly higher platforms, and allow passage from car to car at upper-deck level.
The north eastern US can accommodate split level (double deck) cars only if they are no higher than 14'-6" or 4420 mm. These double deckers run on the Long Island Rail Road: : and on New Jersey Transit: : (contact Bombardier Transportation: ) due to the size of the loading gauge (i.e. bridges, tunnels, etc. are too low). The design found on the Long Island Rail Road, and soon on New jersey Transit, are based on a 1930's Pullman Sleeping Car design for the Pennsylvania Railroad called a Duplex Sleeper. This design provided 24 Roomettes on two levels with the lower level depressed between the trucks. This idea was copied in 1947 for the Long Island Rail Road, making use of a standard P-70 that was electrified. The new cars have the entire center sill lowered to the minimum level between the trucks, providing a depressed floor on that level. The upper level is stacked on top between the trucks. At each end, the a common floor is located in the normal position, with four doors on each side, two of which can be reached only at high level platforms, while the other two are in the normal Vestibular position, with stairs to reach low level platforms. Similarly the structure gauge of the Mount Royal Tunnel limits the height to 14'-6" or 4420 mm .
United Kingdom
In countries such as the United Kingdom, the railway system cannot accommodate double-deck trains. A modest attempt at double decking was made in 1948 on the Southern Railway with the two trains of the Bulleid 4DD class. Although innovative, with stepped compartments, where the bottoms of the upper seats are above the heads of the people on the lower level, but the feet of the people above are not, see , the loading gauge severely restricted their use and they were removed from service in 1971.
Finland
In Finland, VR started operating double-decker sleeping cars on February 1, 2006. The two-bed cabins on the upper deck have toilets and showers while cabins on the lower deck use shared ones. VR also operates double-decker Inter City trains with at-seat power supplies for laptops.
Australia
In 1964, Tulloch Limited built the first double decker trailer cars for use in Sydney. They ran with single deck electric motor cars. The first prototype double deck motor car was built by Comeng in 1969 and production versions entered service in 1972. All CityRail electric commuter trains in Sydney are now double deck. They all have two doors per side per carriage, with a vestibule at each end at platform height. Well-known examples of these trains are the Tangara and Millennium trains. The Sydney double deck commuter trains are 14'-4.5" or 4380 mm high.
The then Public Transport Corporation in Melbourne ordered a prototype Double Deck Development and Demonstration train in 1991. It suffered frequent breakdowns and spent long periods out of use. It was finally withdrawn in 2002 and scrapped in 2006.
Other countries
Indian Railways operates intercity trains between Mumbai and Surat, and Mumbai and Pune .
In the Netherlands, there are two types of double-deck trains, the DDM and the DD-IRM, also called Regiorunner: see Trains in the Netherlands.
In Spain several lines of Cercanías (Renfe's commuter rail service) use double-deck trains.
In intermodal freight transport, many modern types of container well cars cars are designed to accommodate "double-stacking."
Gallery cars
Because of the two levels being separate on most cars, there is a physical limitation on the conductor, as he cannot verify, collect payment and sell tickets to such a large concentration of passengers in one car on each level, owing to the short distance between stops.
The ingenious solution came in the form of the design of the "gallery" car, which featured upper levels, which were really mezzanines running along both sides of the car, with an open area between the mezzanines, hence the term "gallery". This enabled the conductor(s) walking along on the lower level to easily reach up and punch/validate tickets of the passengers seated on the mezzanine level.
Passengers would simply place their tickets in clips along a lengthwise panel, located slightly above the conductor's head and within easy reach. The conductor would then quickly check tickets and move to the next car.
An example is the cars provided and leased in the U.S. by Midwest Transportation & Development Corporation of Chicago. They are of a design proven in service and steadily refined since their introduction in the 1950s. These cars, known as "bilevel gallery cars", are among the most successful designs developed, and are currently in daily use in Chicago, San Francisco, and Montreal (Agence métropolitaine de transport). They provide high capacity (155 to 169 passengers each) and use standard, off-the-shelf components, without relying on proprietary, expensive and hard-to-get replacement parts. Chicago's commuter rail system Metra is currently receiving new versions of these cars and Caltrain, the San Francisco area commuter rail authority, has recently overhauled its fleet.
Another advantage of bilevel gallery cars is the relatively low first step of the vestibule entrance to the car, which is 14 5/8" (371 mm) above the head of the rail. The advantage of this is that commuter rail operators do not have to spend scarce funds on building high-level platforms; a low-level platform is all that is necessary, at a far lower cost.