How Intercity Railways Learned from Metros: Part 5

Push-Pull Operation

The use of multiple unit operations on mainline railways was, for many years, limited to a few suburban routes around major cities. One exception in Britain was the Southern Railway, which installed electric traction on their lines south of London to the towns of Portsmouth, Brighton, Eastbourne and Worthing along the Channel coast in the 1930s. But generally, as far as intercity operations were concerned, locomotive haulage ruled. However, a pointer towards a different form of multiple unit operation appeared in the late 1950s. This was the concept of 'push-pull'.

Push-pull operation was really only an adaption of the multiple unit principle but applied to a locomotive powered train. The train was formed with a locomotive at one end and a driving cab on an unpowered vehicle at the other end. You could do it cheaply by using existing coaches and providing them with a through control cable to connect the controller on the unpowered vehicle at one end with the power system on the locomotive at the other end. With the locomotive at the front, the train was being pulled, with the locomotive at the rear it was being pushed. See more in Development of Push Pull Operation in the UK by Chris Grace.

Figure 8: A push-pull train on the UK's East Coast main line with a non-powered driving car,  known as a Driving Van Trailer (DVT) leading the train. The locomotive is at the rear, pushing the train, thus power is at the rear but controlled from the front. The photo was taken at Finsbury Park by Andy's UK Train Page.

Push-pull operation using multiple unit principles first appeared in France in 1958 and then in the US in the following year. The idea was then gradually adopted in various countries in two forms. In its original form, as described above, uses a locomotive at one end and a coach equipped with a driver's cab at the other end. The number of vehicles in between them may be varied seasonally if required but the formation is not normally varied on a train by train basis.  In the UK, the driving coach is designated a Driving Van Trailer (DVT). It is used to carry luggage and passengers are not permitted to ride in it at speeds over 160 km/h (Figures 8 and 9).

Figure 9: A diagram of the layout of a push-pull train in a terminal. The train arrives with the DVT at the rear and it departs with the DVT at the front. 

The second push-pull form uses two locomotives, one at each end of the train. This was applied to the Channel Tunnel "Le Shuttle" trains and has also appeared elsewhere, notably in Taiwan. The two locomotives are necessary in these cases to provide sufficient power. 

High Speed Multiple Units

The modern two-locomotive concept for push-pull operation first appeared in 1959 with the UK's Blue Pullman series of trains (Figure 10). A diesel power car was provided at each end of a six- or eight-coach set. The concept was further developed in the 1970s with the UK High Speed Train (known as the HST and not to be confused with the new line being built between London and Birmingham) and in France with the TGV (Train à  Grande Vitesse).  The former is diesel powered, the latter electric but the concept is the same. Both these trains employ a power unit at each end with a set of passenger carrying coaches in-between. The Germans joined the club with their ICE train. The only real difference between these trains and the original push-pull concept is that the newer trains were purpose built.

Figure 10: The concept of a power car at each end of a train first appeared in Britain with the Blue Pullman. A diesel Power car was provided at each end of the train. This photo shows one at Paddington in London in 1962.  Source: David Christie Flickr.

While the Blue Pullman concept struggled commercially, partly because all the coaches were to a luxury standard and priced accordingly and partly because of falling traffic, the HST was a standard long-distance train that reduced journey times. It was a huge success. Also, we should not forget the Japanese high speed train concept; they were the first to introduce over 200 km/h running on a regular basis and have kept at the forefront of high speed train technology largely ahead of their German and French counterparts. However, the Japanese HSTs have always been multiple units in the original sense, having many power cars distributed along the train.

Figure 11: In the early 1970s, the high-speed train (HST) appeared in Britain. This was  designed to run at 125 mi/h and was made up with a diesel locomotive at each end of a set of coaches. Here, one is seen at Cardiff Central. Photo: Wallhere.com

The HST name was first used for diesel multiple unit passenger train developed in the UK for 125m/hr running. It is now generally accepted as the definition for any passenger train scheduled to run at over 200 km/h.  For more information on the different types, see the Railway-Technical Website High Speed Train Page.

The next part of this series, Part 6, looks at the concept of fixed formations, length variations and uncoupling.

How Intercity Railways Learned from Metros: Part 4

It Started with a Dream

In America, on 26 July 1897, for one 10 year-old boy a dream came true. He got a brand new train set to play with. This was unusual in those days, since toy trains were rare and expensive but this was no toy. This was a full size train. He was given the controls of the first 6-car multiple-unit electric train to be put together at the General Electric Company’s works in Schenectady, New York and he was allowed to drive the train along the test track in front of the assembled officers and engineers of the Chicago South Side Elevated Railway. 

The boy’s name was Desmond Sprague and he was the son of Frank Sprague, who we saw in Part 3 introduced the first successful electric streetcar system in Richmond, Virginia in 1888. Desmond was showing off his electrical engineer father’s new system of multiple unit control, which Dad was trying to sell to the Chicago South Side Elevated. Well, sell it he did. The demonstration was a resounding success (“Hey, it’s so simple, even a kid can drive the  train”) and, within twelve months of this demonstration, the whole of the Chicago South Side Elevated system had been converted from steam to electric traction with 120 multiple unit cars (Figure 6). Oh that we could do things like this in such a short time today.

Figure 6: one of the Chicago South Side Elevated Railroad electric cars on test in 1898. This was a multiple unit car operating singly. Other cars were attached as required to make up trains.  Sprague himself can be seen second from the left standing behind the motorman. As was standard for many years, American driving positions were in a little cubicle in one corner of the end platform. The rest of the riders were company officials and other assorted hangers on.

Multiple Units

The principle of the multiple unit train was simple. A train could be made up of any number of cars. Each car was equipped with two or more motors mounted on the axles and, like a street car, a controller was provided on each vehicle for the 'motorman'. Additionally, a multi-core cable was was strung along the train and connected to these controllers so that they would all operate under the control of the motorman on the front of the leading vehicle. With this arrangement the train was operating "in multiple".

The arrangement allowed trains of varying lengths to have equal power and turnrounds at terminals were simple. The crew just had to change ends. Trains could be lengthened or shortened during the day by cutting or adding cars as necessary. The idea was an immediate success and quickly spread to other systems, first arriving in London in 1901 on the Central London Railway, now the Central Line of the Underground system.

In later versions of this system, not all trains were made up of motor cars (Figure 7). Some unpowered trailers were sandwiched between motor cars and various combinations were made up but the principal of the remote control of cars from one position remained the same. 

Figure 7: A diagram of a terminal on an urban metro syste (Central London Railway) showing the arrangement of multiple unit traction. It shows a six car train with a motor car at each end containing the traction equipment and driving cab. Because of the small size of the London tube blinds, the early trains built for the system had some of their equipment above the floor. Normally, multiple unit trains built to the original Sprague concept had all the equipment below the floor or under the seats.

Sprague’s conversion of the Chicago South Side Elevated Railroad to electric traction was not without its troubles. During the early months of 1898, while the newly converted trains were being delivered and tested, one of the cars caught fire. It was quickly found that the cause was electrical and that a large batch of the resistor grids were defective. Seventeen of the 20 cars by then delivered had to be taken out of service that day. Suffice to say, teething troubles with new rolling stock are not unique to modern times.

Main Line Reaction

Although urban railway systems quickly adopted multiple unit electric trains, mainline railways were slow to react. This was understandable. Electrification was (and still is) expensive and mainline railways would have to do it over long distances and they would have to convert large fleets of trains. They were already invested deeply in steam locomotive technology and conversion didn't make commercial sense unless the railway was very densely used. Even the improved terminal operations of electric railways weren't enough to convince most mainline railways to get into wholesale electrification.

In Part 5, we look at how it took until the late 20th century for main line railways to fully realise the benefits of metro operational techniques.

How Intercity Railways Learned from Metros: Part 3

Electric Traction

The development of electric traction for railways started seriously in the 1880s. Before this time, it was confined to a range of experimental schemes that were largely unsuccessful. The driver behind the development of electricity as a prime mover for rail vehicles was the need to improve urban systems, specifically streetcar or tramway operations. These were using horses. Horses were (are) expensive to keep, you needed three pairs a day for each vehicle you operated and they produced a lot of waste. In London alone, over 400 tons of manure was produced each day by horses in the late 19th century and it all had to be collected and disposed of. 

However, electrifying the horse-drawn tramways was expensive. You had to build your own power station because there was no national grid in those days, you had to provide a system of supply along the line, usually an overhead wire and you needed to buy new vehicles, complete with electric motors and the associated control equipment. But this expense could be justified if there were enough people using the system and, in the developing urban context, there were. Cities were expanding rapidly, suburbs were being built around them and people needed to get from them to their work and back each day. The urban railway had arrived. 

Figure 5: Richmond Union Passenger Railway Company showing off one of their original electric street cars introduced in 1888. The trolley pole on the roof of the car connected to an overhead line carrying the electrical supply to the the controller at the car end and thence to the motors under the floor. The return circuit was made through the running rails at zero volts. The driver or 'motorman' is at the right hand end and the conductor at the rear. Photo: Collection of: Frank J Sprague papers, OVERSIZE PHOTOGRAPHS. Image ID: 5665668

Electric Streetcars

The first successful electric tramway was opened in Richmond, Virginia in 1888. The engineer was Frank J. Sprague, a former US naval officer who believed in the future of electricity as a means of powering railways. His Richmond streetcar (Figure 5) carried 40 passengers and a crew of two. Controls were provided at both ends of the car so that, at the terminals, to change direction, the crew merely swapped ends. 

As urban tramways spread across the US and were introduced into Europe and other big cities around the world, increased patronage drove moves to increase capacity by lengthening a single street car into a train. The leading car was powered but it could drag additional, unpowered cars behind it.  But (isn't there always a 'but'?), with this, the terminal problem of the trapped motive power returned and powered cars had to be exchanged at terminals in the same way as the locomotives we saw in Part 2 of this series. Some lines had loops built to avoid the terminal swapping.

Then, in 1897 in America, our Mr. Sprague demonstrated an electric train using multiple unit control.  This trial was for one of Chicago's elevated railroads - a network of steam powered urban lines - and he was trying to persuade them to adopt electric traction. Part 4 of this series explains how it worked and what happed next.