A Staff Report from the Straight Dope Science Advisory Board

When multiple locomotives pull a train, why are they often pointing in opposite directions?

March 11, 2003

Dear Straight Dope:

I moved into an apartment building near railroad tracks and I've noticed that the typical train has two or three engines pulling it. I can see where the extra engines would give additional power, but the second and third engines are nearly always oriented backwards from the lead engine, which (seemingly) means that the other two engines are running in reverse. Why not have all engines pointed in the same direction?

Backwards is just a state of mind, G. Who's to say that your "backwards" locomotives aren't really facing forwards and that it's the rest of the train that's backwards? Okay, so that's stupid. But it's also likely to be true . . . for the return trip.

There is a limit to the amount of power you can get from a reasonably sized locomotive, even if the limit is pretty high. Long trains usually need more than one locomotive.

A "locomotive consist" is a number of locomotives coupled together, all controlled from a single cab by a small crew. In railroad parlance, individual locomotive are called units, and operating several units in tandem with one set of controls is called MU (for "multiple unit") operation. Locomotives come in different configurations.  Two-cab units (with a cab at either end) exist, but they're unusual in the U.S. Some units (called "drones") have engines and motors but no cab of their own, and have to be controlled from another unit.  Others (called "slugs") have cabs but no engines. But the usual U.S. freight locomotive has a single cab, a diesel engine, an electric generator and traction motors to turn the wheels.

When assembling a locomotive consist, it pays to think carefully about how you arrange the units. Modern diesel-electric locomotives run just as efficiently in either direction, and it's not as if there's anybody actually in those backward-facing locos trying to drive while looking over their shoulders. (Unless Captain Hazelwood has taken up a new profession). You should obviously have the cab of the front locomotive (the "lead unit") facing forward so the crew can see where they're going. What's not so obvious is that it's also a good idea to have the rearmost locomotive with its cab facing backwards. Why? Because that way, after the cars are uncoupled at the end of the line and shunted away, the locomotive consist can now move in the opposite direction, perhaps with a new train attached at the other end. Reversing direction this way is a little more complicated than the crew just hopping in the back and taking off, but turning a few valves and flipping a few switches is usually more convenient than physically turning the whole consist around.

Pairs of locomotives work so well together that they are often used as a unit, semi-permanently united in "married pairs." (As in all marriages, they never see eye to eye.) That way, the pneumatic and electrical connections between them don't need to be messed with each time they start a new run. When you see three locomotives in tandem, chances are good that two of them (the ones that are back-to-back) are a married pair, and the other one is a third wheel, so to speak. Occasionally you'll see a pair of switch engines coupled together in this way, one with a cab and one without. Such a combination is called a "cow and calf," the cow being the unit with the cab, and the calf the drone.)

I don't want to leave the impression that there is no way to physically turn a locomotive around. A "reverse loop track" lets you make something like a U-turn (except it looks more like a P than a U) and a "wye track" lets you make a three-point turn. Either one takes up a lot of room (especially the loop), so they're not always available near where the crew wants to turn around. The turntable was an ingenious invention that physically rotated the locomotive (see picture), and was a necessity in the days of steam engines, which were seldom if ever operated in reverse for extended distances in mainline service. But it's not very practical for turning consists. Besides, there was more than one case of locomotives falling into the turntable pit and being wrecked. So now we know that train wrecks in the Pit are nothing new.

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