MU and Push-Pull in Europe

MU and Push-Pull in Europe


By Ernest H. Robl


In the U.S., where many as five or six large diesel locomotives on the front of a heavy and long cross-country freight train have long been the norm, the fact that all of these locomotives are operated by a single engineer has also long been taken for granted.

On the other hand, while push-pull passenger trains – where the train operates in one direction with the locomotive pushing, controlled remotely by the engineer in the leading car – have existed in Europe for many years, large-scale multiple-unit (MU) operation of locomotives is a fairly recent development.


That’s not to say that some heavy European trains have not had more than one locomotive up front. But, until recently, most of these trains have been double-headed.

Double heading is having two (or more) locomotives up front, each operated by its own engineer. That was the case all the way back into steam days, when there was no such thing as MU control.

But, for the most part, that practice continued in Europe well into the modern diesel and electric era.

MU control – used both for a single engineer operates multiple locomotive units or for operating a locomotive from the opposite end of the train – uses just a single crew member to control all the motive power.

Advantages and disadvantages

In this piece, we’ll look at how and why the use of MU operations developed differently in the U.S. and Europe.

Both double heading and MU operations actually have their own advantages and disadvantages. These focus on the fact that you will need more horsepower on the steepest segment of a route and less on the flatter parts of a journey.

With double heading, the major disadvantages are that you will need a second crew and there is he process of coupling and uncoupling. The advantage is that, because the second engine is manned, it can cut off and return to its base – or assist other trains in the opposite direction) once it is no longer needed on the first train.

With MU, you only need one crew, but you are also running more locomotives than you need on part of your journey.

In the U.S., diesel engines in a multi-engine consist are described as on-line, off-line, or dead in tow (DIT). If an engine is on-line, it responds to commands from the lead locomotive. If it is off-line, it is still running (idling), but not exerting any effort. A DIT locomotive is shut down and is just another (heavy) car being pulled along.

In German, equivalent terms are “mit Leistung” (with effort); “ohne Leistung” (without effort); and “kalt (cold)—a term that goes back to the steam era, but which is still used when a diesel or electric locomotive is hauled in non-operating condition.

In the U.S., on some long-distance routes, freight railroad take some units off-line when they are not needed to conserve fuel and then put them back on-line as they are needed. This can be done either remotely from the lead locomotive (requiring specially installed equipment on all units) or by having a crewmember go back to the unit in question and reset its main controls. Because starting a large diesel engine (the prime mover or actual motor) is not a simple process, locomotives are seldom completely shut down in transit unless there is a mechanical or electrical problem that requires such a shutdown.

Different paths

In North America, since diesel traction came into its own in the second half of the 20th century, there have been only a limited number of locomotive manufacturers. These quickly realized that they could sell more diesels to the large railroads if their units were compatible with those of other manufacturers.

The good thing is that North American MU standards were developed early. The bad thing is that these standards are now outdated, but are also difficult to change because of the large number of locomotives in operation with these MU standards.

In Europe, on the other hand, through the late 20th century, most countries tried to have their own locomotive manufacturing companies—which built locomotives specifically for that country’s railroad. (Until the late 20th century, most countries only had a single national railroad, though some smaller operators did exist all along.)

With different electrical and signal systems in different countries, locomotives seldom crossed borders. Austria and Germany were one exception, with the two countries having common electrical and catenary standards, as well as compatible signal safety systems. Switzerland, while using the same voltage as Austria and Germany, had different catenary standards, and any Austrian or German locomotives going into Switzerland had to (and still have to) have special pantographs.

So, in Europe, with the wide variety of manufacturers, each developed its own set of controls for handling the output of the main transformer or diesel motor. As mentioned earlier, push-pull operations came fairly early to Germany, but in these cases, the train had to have a cab control car that exactly matched the motive power being used — and, of course, all cars in the consist had to be cabled for those controls..

At the same time, European freight trains have always been shorter, while, by late era IV, one European electric locomotive had the pulling power of two to three American diesels.

Diesel and electric railcars in Europe generally had MU capabilities by mid 20th century, but, again, these units were usually only capable of being operated from cab control cars or other powered cars of the same family.

Reasons for incompatibility included the fact that different families of EMU or DMU had different features, such as mechanically operated doors, on-board loudspeaker systems, etc., and that therefore different types of cables were needed to connect each type of equipment.


Near the end of the 20th century, several factors pushed European railroads toward much wider implementation of MU controls for locomotives:

  • Both freight and passenger trains were getting heavier, meaning that they needed more horsepower over most of their journey.
  • Railroads became painfully aware of the labor costs required to couple and uncouple equipment and to run locomotives around passenger consists at the end of their runs.
  • Railroads also became more interested in being able to use passenger rolling stock for a variety of services, eliminated many train sets designated for specific types of routes or operations.
  • Particularly within the European Union (EU) railroads found it more logical to buy off-the-shelf motive power, rather than designing their own locomotives.
  • Locomotive-leasing companies saw that they could fill a market need by offering motive power on a short-term basis to both national railroads and private operators. But, these locomotives had to be compatible with existing railroad equipment and usable in a wide variety of services.

Then, the ability to go to a mostly universal MU standard was simplified by one thing — robust computers aboard locomotives.

Remember, early MU implementations used connecting cables with lots of wires, each of which carried a specific signal.

Today, MU operations largely rely on a standard data bus, with very few wires. These cables are already wired into almost all passenger rolling stock.

All modern locomotives already have some type of computer on board that manages most of the locomotive operations. Now, that computer could be adapted to translate specific operating commands, such as throttle settings, into a generic command that is sent on the data bus. Any second locomotive operating in MU simply has its computer “listening” for applicable generic commands, which it then translates back into commands for its specific onboard equipment.

The same is true for cab control cars. They send out generic throttle and other operational commands, which the computer on the locomotive then translates back into commands specific to the locomotive.

This even means that the same cab control car can be used with either electric or diesel motive power on the other end of the train. The cab control car is even capable of sending out commands for a locomotive to raise or lower its pantographs – commands that would, of course, be ignored by a diesel locomotive as they are not applicable.

The remote locomotives can also send back a variety of status indications, which are displayed on the operating console being used by the engineer, whether in another locomotive or a cab control car.

With this system, it is even possible to MU diesel and electric locomotives together.

Not only were most diesel and electric locomotives built during the past two decades delivered with universal MU controls, but many older locomotives were rebuilt for universal MU when they underwent major overhauls. For example, in Austria, the series 1142 and 1144 are upgraded versions of 1042 and 1044 locomotives that now have MU capabilities, among other improvements. (The locos retained their individual number; only the series number was changed during the upgrade.)


An important factor in the use of multiple diesel or electric locomotives on a heavy train is that these additional locomotives not only provide more pulling power on uphill routes but also provide more braking effort on downgrades.

Both diesel-electric and straight electric locomotives can turn their traction motors into generators feeding power to a resistance grid for so-called dynamic braking. Dynamic braking reduces wear on brake shoes on both locomotives and other rolling stock and helps avoid such problems as overheated brakes on long descents.

Diesel-hydraulic locomotives can also use their transmissions for a form of dynamic braking, much the same as when you put an automatic transmission car in a lower gear.

This is one of the reasons why you will now seldom see helper locomotives cutting off at the apex of a route —unless that location represents a national border where a new electrical system goes into effect.

Implications for modelers

  • If you model Era IV or earlier diesel or electric push-pull operations, you need to keep an eye on appropriate matching rolling stock and motive power. Not all locos would work with various rolling stock and control cars..
  • The same goes for early 20th century EMU or DMU operations.
  • If you model modern MU operations, you again need to look at appropriate motive power combinations. In Austria, an 1144 and a 1016 can be run in MU. A 1042 and a 1016 cannot be run in MU because the 1042 in not equipped for such operation. You can still double-head non-compatible locos, but routine double heading is fairly rare in the 21st century. (Around the turn of the century, some older classes, such as the 1110.5 series were specifically designated for helper service. And, in this case, the 1110 designation does not indicate MU capability.)
  • For modern push-pull trains, any MU capable locomotive can be used, but do look at the engine class. In Austria, a 1016 (or an 1116 or 1216) Taurus would be overkill for a 3-4 car local push-pull set. An 1142 would be a better match. And, a 2016 diesel, with less output than similar-sized electrics, would still be appropriate for even a modest-sized push-pull train.
  • You also want to make sure that any locos that you run together — whether in MU our double-headed — are geared approximately the same so that the locos will work smoothly with each other and not struggle against each other.
  • You can use unpowered dummy locomotives to simulate multiple traction, but with an unpowered dummy, you are simply adding more weight to a train. A better use for these dummy locomotives is at the rear of a train as a simulated pusher, when you already have a couple of working locos up front. (Powered pushers are usually problematic on model railroads, especially if you have any light cars in your train.)


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