MU (Multiple Unit) Connections are a small but important part of a diesel electric, or electric locomotive. These electrical and pneumatic cables connect multiple locomotives in a consist to allow control of all from the cab (control room) of one unit.
MU connections, or hoses as modelers often call them, have been around since the early days of electric and diesel locomotives. This was one of the efficiencies that helped make these new technologies more efficient than steam. Using MU hoses, railroads could use smaller diesels to effectively build a single unit capable of moving the train at hand. The ability to run four or more locomotives with one crew improved train handling and offered a tremendous labor savings.
Interestingly, many railroads ordered their early MU locomotives semi-perminently connected by drawbars instead of couplers or numbered as "A-B-C-D" etc. so that they would appear on the record as one unit. This was due to concerns that labor unions would require seperate crews on all locomotives. That was a fight that was never brought however and most of those sets were later broken up and freely mixed.
Although MU connections were sold as an option early on, by the 1950s they were standard equipment on just about everything. Some railroads did opt to save expense on switching locomotives that were never expected to need additional connections.
It was also not uncommon for railroads to later modify "cab" units like EMD F and E classes or ALCO FA and PAs with additional connections on the noses of the locomotives to allow these engines to be placed in the middle of larger consists. It was clearly a move of operational efficiency over aesthetics.
How They Work
There are two types of MU connections. First, there are pneumatic hoses. These control the locomotives' air brakes (main resevoir, actuating and application/release) and sand. These are normally found on the pilot of the locomotives on either side of the coupler along with the larger trainline air hose(s). Originally there were eight hoses (four pair). Today six are standard, with the sanding pair eliminated (this is now automatically deployed on each engine).
In addition to the air connections, a larger electrical cable is also used for power control. When not in use, the other end of the cable is usually plugged into a "dummy" receptical on the pilot. These are typically found above the coupler in the center of the locomotive.
When connected, these hoses and cables transmit signals from the control stand in the lead or controlling unit to all other engines. A knob on the control stand allows crews to set up the locomotive to control or be contolled. In normal practice, the lead engine is set up as the control unit, but it does not have to be.
Today more and more model locomotives are coming with these details already applied by the manufacturer. For those that aren't, appropriate detail parts are made by several manufacturers in just about every scale. Plastic and metal parts can be found and are easy to add to models.
The common detail parts model the connections in their unattached position, dangling or plugged into a dummy connection. Most modelers do not model connected hoses between units as it complicates operation on tight curves and makes changes to the locomotive consist very difficult. You can however simulate complete MU connections using thin wire carefully attached to both locomotives if you never intend to separate them.
You can also make these wire connections an active part of the model. This can be done to improve electrical pickup or to use a "dummy" or unpowered locomotive to house a decoder, possibly with a sound speaker, when adding DCC to smaller engines. These can be a little more difficult to install. Just make sure you leave adequate wire slack for curves.