MUed locomotives

Bibliophage has it mostly right, but I do want to poin out a couple of inaccuracies.

In North American parlance, a locomotive with a prime mover and no cab is a B unit, not a drone. Slugs may or may not have cabs. CSX runs phosphate trains in Florida with cabbed slugs; Conrail used cabless slugs to switch their major yards. A calf is a B unit of a low carbody switch engine

Matched pairs was a common practice until the early '60s, especially with passenger power. Today, when motive power can and does wander the country in power sharing agreements, the practice is almost unheard of. That doesn’t mean that a consist can’t remain intact for weeks at a time, however.

Model railroader and railfan since I was in single digits…

Slugs are simply locos without prime movers. They’re typically connected back-to-back with a prime mover. They may or may not have cabs.

A units are locos that have controls. B units are locos without controls, and need to be MUed to an A unit. Sometimes, an MU consist will be built with an A unit slug in front and a B unit prime mover. The crew rides in the forward A unit and enjoys a quieter ride than if the prime mover was 10 feet away from them.

For further amusement, there’s a kind of loco called a “robot” which is placed in the middle or end of a long train to provide added power and/or control. They can be controled by wire or by radio, (aka “Locotrol” and are more often than not, unmanned.

Prime movers, for the unaware, are the (usually) diesel engines that turn the generators which then power the electric traction motors to turn the wheels. It’s a lot simpler to control electricity to motors than engine power to wheels via clutches and transmissions.

Traction motors are built into the loco’s trucks (wheels). Here’s a pic of a traction motor: