Push–pull is a mode of operation for locomotive-hauled trains allowing them to be driven from either end.
A push–pull train has a locomotive at one end of the train, connected via some form of remote control, such as multiple-unit train control, to a vehicle equipped with a control cab at the other end of the train. This second vehicle may be another locomotive, or an unpowered control car.
In the UK and some other parts of Europe, the control car is referred to as a Driving Trailer (or Driving Van Trailer (DVT) where there is no passenger accommodation); in the USA and Canada, they are called cab cars.
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Historically, push–pull trains with steam power provided the driver with basic controls at the cab end along with a bell or other signalling code system to communicate with the fireman located in the engine itself in order to pass commands to adjust controls not available in the cab.
At low speeds, some push–pull trains are run entirely from the engine with the guard operating bell codes and brakes from the leading cab when the locomotive is pushing the train.
Many mountain railways also operate on similar principles in order to keep the locomotive lower down than the carriage so that there is no opportunity for a carriage to run away from a train down the gradient and also so that, if the locomotive ever did run away, it would not take the carriage with it.
Modern train control systems use sophisticated electronics to allow full remote control of locomotives. Nevertheless push–pull operation still requires considerable design care to ensure that control system failure does not endanger passengers and also to ensure that, in the event of a derailment, the pushing locomotive does not push a derailed train into an obstacle worsening the accident. The 1984 Polmont rail crash (in Scotland) occurred when a push–pull train struck a cow on the track. Push–pull operation has also been blamed for worsening a number of derailments by trains of the Metrolink commuter rail service in greater Los Angeles.
When operating push–pull, the train can be driven from either the locomotive or the alternative cab. If the train is heading in the direction in which the locomotive end of the train is facing, this is considered 'pulling'. If the train is heading in the opposite direction, this is considered 'pushing' and the motorman or engine driver is located in the alternative cab. This configuration means that the locomotive never needs to be uncoupled from the train and ensures fast turnaround times at a railway station terminus.
In certain situations, the locomotive is placed in the middle of the train rather than at one end but driven from cabs at the train ends. The Great Western Railway sometimes did this when multiple autocoaches were linked up in an autotrain, as the mechanical linkages used to control the steam locomotive were not capable of reliable operation through more than two interconnections.
The Swiss Railways sometimes run with the locomotive in the middle during peak times, when additional carriages, generally including one with a cab, are added to a train where the locomotive was previously at the end (more commonly these carriages will however be added on to the cab end).
When the locomotive is placed mid-train, both directions are considered 'push'.
If a rake of coaches has a driving cab at each end, a locomotive could be attached to either end. This is useful if different locomotives are desirable for two sections of the route, but shunting needs to be kept to a minimum and preferably only at a single intermediate station.
For example, where electrification ends a diesel locomotive is required for the non-electified portion. The train commences being driven from the front cab with an electric locomotive pushing. At the end of the electrified section of line, it is coupled onto the back of a diesel locomotive waiting at the platform track. The electric locomotive is detached, and the diesel locomotive pulls the train for the rest of the journey. For the return trip, the diesel locomotive pushes the train until the start of the electified section, whereupon the train is coupled onto the back of a waiting electric locomotive, which then pulls the train (without the diesel locomotive) the rest of the way.
This practice was used with the London Waterloo to Weymouth trains from 1967 to 1988 when the electrification ended at Bournemouth. Rather than an electric locomotive, however, an especially powerful British Rail Class 432 4 car electric multiple unit was used between London and Bournemouth.
Alternatively, a push–pull train, especially a long one, may have a locomotive on both ends so that there is always one locomotive pushing and one locomotive pulling. In this case, caution must be used to make sure that the two locomotives do not put too much stress on the cars from uneven locomotives. This two-locomotive formation is used by the InterCity 125 (and its Australian equivalent, the XPT). It is usual to arrange things so that the trailing locomotive supplies less power and that the locomotive at the front does more pulling than the locomotive at the rear does pushing. Having an independent locomotive as opposed to a power car at each end is also known in the railway world as a top and tail. When this configuration is used in the US, only one locomotive is allowed to provide head end power (HEP: electricity supply for heating, lighting, etc.) to the train.
In this configuration locomotives hauling a train are located other than at the front or back. It may include remote control locomotives in the middle of a train. Where operational considerations or economics require it, trains can be made longer if intermediate locomotives are inserted in the train and remotely controlled from the leading locomotive.
The first company to use the system was the Great Western Railway which, in 1904, equipped carriages and 0-6-0 locomotives as an "autotrain" to run on the Brentford Branch Line (between Southall and Brentford) as an experimental substitute for steam railcars. Control was by rodding and the mechanism allowed the driving compartment to be either one or two carriages-distant from the engine. With the engine in the middle of a formation, up to four carriages could be used. To reduce the surprise of a locomotive at the "wrong" end of its train, some were initially fitted with panelling painted in carriage livery.[1] The experiment was successful and the company's remaining railcars were gradually converted for autotrain use and purpose-built units constructed. Other companies followed the lead in 1905: the North Eastern and LBSCR using a compressed-air method of control and the Midland, using a cable-and-pulley mechanism. The Great Central deployed the trains in 1906, using cable controls similar to that of the Midland. By the 1920s most companies had them and they remained in use until replaced by diesel multiple units (DMUs) in the 1950s.[1]
In 1967, the Southern Region, already familiar with operating electric multiple units, applied the technique to its services from London Waterloo station to Bournemouth, which were operated by electro-diesel locomotives.[1]
In the early 1980s, the Scottish Region trialled a system using a Class 27 locomotive at each end of a rake of coaches that had been specially retro-fitted with the necessary 'Blue Star' multiple working cables to control the remote unit; but some problems of delay in actuation were experienced. They were replaced by a system whereby a Driving Brake Standard Open (DBSO) could control the Class 47/7 locomotive via computerised time-division multiplex (TDM) signalling through the train lighting circuits. This had the added benefit that intermediate carriages needed no special equipment and was found more satisfactory. Such trains became widely used on the intensive passenger service between Edinburgh Waverley and Glasgow Queen Street.[1][2]
In 1988, Mark 3 Driving Van Trailers (DVT) were built for the extended electrification of the West Coast Main Line and the Mark 4 DVT was built as part of the Electra project for the East Coast Main Line. The Mark 4 DVT can be retro-fitted to tilt.
More recently, some of these DVTs have been modified to operate with Class 67 locomotives for the Wrexham, Shropshire, Marylebone Railway Company.
CIE's first push–pull trains were conversions of their 2600 class DMU (Park Royal body, AEC motors) running with the original 201 class Metropolitan Vickers Bo-Bo diesels re-engined with EMD 567 prime movers; the cars were subsequently renumbered in the 6100 series (driving van trailers), 6200 series (trailer with "blind" cab end) and 6300 series (double-gangway intermediate car). In push–pull formation, they operated the Dublin Suburban rail services from 1971 until the inauguration of the DART EMU service in 1984. The remaining push–pull trains operated on Dublin-Maynooth commuter services until they were supplanted by Cravens, and later by the newer 2600-class "Arrow" DMUs.
Iarnród Éireann employs push–pull trains of three different kinds. The first of these were built in 1989 and are based on the British Rail Mark 3 design, with a non-gangwayed driving cab fitted.[3] These are operated with 201 Class locomotives, although in the past 121 Class locomotives were also used. It was not unknown for these sets to be hauled as normal coaching stock by non–push–pull fitted locomotives. The sets originally operated in the Dublin outer-suburban area and on the Limerick–Limerick Junction shuttle, but were gradually moved to mainline Intercity routes out of Heuston after the introduction of railcar sets elsewhere.
Also in operation on the IÉ network are De Dietrich Ferroviaire -built "Enterprise" push–pull sets, jointly owned with Northern Ireland Railways for operation on the Dublin to Belfast route. These are powered by 201 Class locos.
The other type of push–pull train used in Ireland is the Mark 4 type (not to be confused with the British Rail Mark 4 type). These sets, delivered in 2005–06, are used exclusively on the Dublin to Cork route, again operated by 201 Class locos.
The Auckland suburban network run by Veolia uses rebuilt British Rail Mark 2 carriages in two formations: three SA-class cars and one SD class car fitted with a cab, with motive power provided by a DC Class locomotives leased from KiwiRail, forming a four-car set. The second formation was introduced in September 2010, which is five SA cars and one SD car, with motive power provided by a DFT\B locomotive, leased from KiwiRail, forming a six-car set.
All SA and SD class cars were rebuilt by Hillside Workshops.
Auckland also operates ex-Queensland SX carriages in push–pull mode with two DBR class locomotives.
The Wellington suburban network operated by TranzMetro predominantly uses EMU's, but does include a single push–pull formation: six refurbished BR Mk 2 carriages (designated SE) sandwiched by a two of the three refurbished EO class electric locomotives and it is presently (October 2011) employed on peak express services on the Hutt Valley Line.
Many commuter services into cities such as New York City, Boston, Massachusetts, San Francisco Bay Area, and Montreal are operated by push-pull services, often with bi-level passenger cars. The Muskingum Electric Railroad was a private, coal-hauling railway in central Ohio that ran for more than 20 years with two driverless General Electric E50C Electric Locomotives that ran backwards from the Coal-Fired Powerplant they served to the mine where their trains were loaded by affixing bogie trucks, a headlight, and a horn to the last freight car on each train.