Prototype Railroading: Multiple Units

Every day in railroading, trains run with more than one engine, yet only with one train crew.  This is accomplished by a "multiple unit" system on locomotives.  It is something that is an important part of railroading, yet very much taken for granted, and never really thought much about unless something goes wrong with the system.  Modern diesel and electric locomotives have the ability to run each other, meaning that only one train crew is needed to run a virtually unlimited number of engines.  Most freight trains run with more than one engine, because it means they can be run longer and heavier, without requiring more employees.  Long haul passenger trains, such as Amtrak trains, often run with more than one locomotive as well, and shorter haul, commuter trains usually have the ability to be controlled from either end, with only one locomotive.  All this is accomplished through a multiple unit system.

In the days of steam locomotives, there was no such thing as a multiple unit system.  Most trains ran with just one engine, and consequently, they ran considerably shorter than today's trains.  When more than one steam locomotive was needed, more than one crew was also needed.  Even identical steam engines responded differently, and so a crew was needed on each one.  The two crews would have to work together carefully to match speed and available pulling power.  This system actually worked pretty well, all things considered, although it was not commonplace to run multiple locomotives on steam trains, because it cost the railroad more.  Typically the railroad prefers to pay multiple crews to move multiple trains, not one train!

As diesel and electric locomotives were developed, the ability to operate more than one from a single location also developed.  This is one way that diesel locomotives had an advantage over steam locomotives.  Three or four of them could be placed on a train, and still only require one crew.  The meant the amount of horsepower could be adjusted to meet the demands of almost any train, and the railroad could run longer trains.  Longer trains mean more money, and yet there is no additional cost for additional crews.

A pair of MU jumper cables.
Photo from www.epowerrail.com.

On every locomotive, diesel and electric, there is an identical plug on each end, called the multiple unit, or MU plug.  There is a standardized MU jumper that fits into that plug.  When locomotives are coupled together, the jumper cable is plugged into the MU plug on each locomotive, spanning the gap between locomotives.  On the leading locomotives, brake valves are cut in, and the control switches are set up to identify that as the controlling locomotive.  On all the trailing locomotives, brake valves are cut out, as the brakes will be controlled from the leading locomotive, and the control switches are positioned to identify those as non-controlling locomotives.  When all the valves and switches are positioned correctly, the trailing locomotives all take commands through that MU jumper cable.  The commands come from the engineer operating the controls of the leading locomotive.

Most of the commands for operating multiple locomotives are sent through the MU jumpers, however there are a few things that are sent through air hoses, and some things that do not get sent at all.  For example, the horn and bell will only sound on the load locomotive.  There really is no need to have more than one engine on a train whistling for crossings.  All it would do is annoy people more.  Other controls, such as the heater or air conditioner in the cab, only applies to the unit on which those controls are operated.  If for some reason the train crew wants heat in the second locomotive, they have to walk back there and turn it on there.  Some newer locomotives also have electric hand brakes, where simply pushing a button applies or releases the hand brake.  Those are also controlled locally, and no information about their condition is sent through the MU cable.

The train brake hose, on the left, and the three independent
brake hoses, on the right.
Photo from www.mdmlocomotiveworks.com

Air brake information is not sent through the MU cable, but rather through a set of hoses.  Locomotives have three brake systems on them.  The train brake, or automatic brake, is the air brake that operates on all locomotives and cars in the entire train.  A hose connects the brake pipe on every locomotive and car from the head end to the rear end, and when the engineer makes an application on the train brake, it effects the entire train.  Locomotives have a second set of air brakes, called the independent brake.  When the engineer uses the independent brake, it only effects the air brakes on the locomotives.  A third braking system, called the dynamic brake actually uses the locomotive's traction system to slow the train down.  The motors, which move the train, can also be used to provide quite a lot of resistance, and slow a train down quite considerably.  Only the dynamic brake information is sent through the MU cable.  The independent brake information is relayed through a set of three air hoses, seen on the left.  The three hoses all relay different information, and connect the independent brake systems on all the locomotives.  Whenever locomotives are coupled together, these hoses, along with the the train brake hose, and the MU cable, must all be connected for everything to work properly.

Some trains operate with distributed power locomotives, which handle things a bit differently.  Distributed power is when one or more locomotives is placed in the middle or at the end of the train, yet still controlled from the leading engine.  This is a newer technology than the MU cable.  Distributed power allows the engineer on the lead locomotive to control all locomotives on the train, via radio signals.  The lead locomotive and the distributed power locomotive communicate commands via radio, which the on board computers transmit, receive, and process.  This operates a little differently than more traditional multiple units.  When the MU jumper is used to connect locomotives, they all do exactly what the lead locomotive does, at the same time.  With distributed power, the engineer has the ability to control the distributed power locomotive, or DP, separately.  The DP can be doing different things than the head end, which allows for better train management and smoother handling.  It can also be set up to do exactly as the head end, if that is how the engineer would like to operate.  the controls for the DP are all managed from a computer screen in the lead locomotive.  All the information, including brakes, is sent via radio to the DP, and there are no mechanical connections running through the train, except for the brake pipe.  This is similar to multiple unit operation, in that it allows for the DP locomotive to be controlled by the engineer on the controlling engine, but different in that it allows quite a bit more flexibility in operations.  Both systems have allowed railroads to put together much longer and heavier trains than would be possible otherwise.  They allow very long and heavy trains to be operated by just one crew, sometimes over a mile away from the end of their train.