Push-pull train

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 multiple-unit train control, to a vehicle equipped with a second control cab at the rear of the train. In the UK the control vehicle is referred to as a Driving Trailer or Driving Van Trailer (DVT), while in the USA they are called cab cars. Alternatively, the train may have a locomotive at each end.

Train formation

Locomotive at one end

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 alternate 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 engineer is located in the alternate 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.

Locomotive in the middle

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 GWR 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. When the locomotive is placed mid-train, both directions are considered 'push'.

Two locomotives

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 less power is supplied by the trailing locomotive 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.

History

Britain

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 its Southall and Brentford branch 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.Simmons, Jack; Biddle, Gordon (editors) 1997. "Oxford Companion to British Railway History" p 407. Oxford University Press. ISBN 0-19-211697-5] 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 railcars in the 1950s.

In 1967 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.

In the early 1980s the Scottish Region trialled a system using compressed air and multiple electric cabling to control the remote unit but some problems of delay in actuation were experienced; a system of computerised multiplex signalling through the train lighting circuits (with the added benefit that intermediate carriages needed no special equipment) was found more satisfactory and became widely used on the Class 47/7 -operated intensive passenger service between Edinburgh and Glasgow Queen Street. [Glover, John (1999). "Railway Operations" p 58. Ian Allen, Shepperton, England. ISBN 0-7110-2689-0]

In 1988, Mark 3 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 in 1988 as well. This can be retrofitted to tilt.

New Zealand

The Auckland suburban network run by Veolia uses DC class locomotives owned by Toll NZ, operating in push-pull mode with sets of 2 or 3 SA cars and an SD driving car (all ex British Rail Mark 2 carriages rebuilt for suburban service on 1067mm or 3' 6" gauge lines) owned by the Auckland Regional Transport Authority (ARTA).

ee also

*Air brake (rail)
*Railway brakes
*Rail terminology

Notes

Further reading

*cite book | last =King | first =Mike | title =An Illustrated History of Southern Push-Pull Stock | publisher =Ian Allan Publishing (OPC) | date =2006 | pages =160 pages | isbn = 0860935965
*cite book | last =Lewis | first =John | title =Great Western Railway Auto Trailers: Pre-grouping Vehicles (Part 1) | publisher =Wild Swan | date =1991 | pages =208 pages | isbn = 0 906867 99 1
*cite book | last =Lewis | first =John | title =Great Western Railway Auto Trailers: Post-Grouping and Absorbed Vehicles (Part 2) | publisher =Wild Swan Publications Ltd | date =1995 | pages =184 pages | isbn = 1 874103 25 9
*cite book | last =Lewis | first =John | title =Great Western Steam Railmotors: and their services | publisher =Wild Swan Publications Ltd | date =2004 | pages = | isbn = 1 874103 96 8

External links

* [http://www.railway-technical.com/pu-pl.shtml The Development of Push-Pull Operation (at Railway Technical Web Pages)] – "history, mainly UK"
* [http://chicago.railfan.net/cgi/photos.pl/?page=AMTK_Cabbage Amtrak cab-baggage (cabbage) cars]