MU Pin Arrangement

[Engineer Spike]

How is it that before the AAR standardized the pin arrangements, each road had its own? Why wouldn't EMD or Alco wire it in a standard pattern from day 1?

[Allen Hazen]

My impression (from hazy memories of a long-ago article in "Trains") is that the manufacturers DID choose arrangements: different ones, of course, and not even constant across the models from a given manufacturer: Alco RS-1, for example, i.i.r.c. had an m.u. pin arrangement incompatible with that used on 244-engined Alcos, and early EMD cab units had a different arrangement from that used on later EMD units.

I think some railroads (U.P. may have been mentioned) tried to cope with this by equipping at least some of their first-generation diesels with more than one m.u. socket, allowing connection to a broader range of types than their manufacturer's standard set-up would have allowed. And I think it was possible for a railroad to special-order something other than the manufacturer's standard in order to (for example) allow m.u. operation with locomotives from another manufacturer already on the roster.

(On a tangent: when British Rail started serious dieselization in the late 1950s, they COULD have learned from American experience and insisted on compatible m.u. arrangements on all their locomotives... but instead they ordered locomotives with several different arrangements of m.u. gear: I think it was common to paint a symbol beside the m.u. socket to mark the type of m.u. possible.)

[EDM5970]

Briefly-

The Alco 539s were 19 pin. Some of the early other Alcos (FAs, RS-2, -3) were 21 pin. I've seen drawings for 16 pin on early EMD switchers. I believe some of the early passenger units (DL-109 and Es) were also 16 pin. I won't even address BLW and some F-Ms with the air throttles. GE 44 tonners could get by with 12?

The manufacturers at first used pretty much the number of wires needed to do the job at hand. There was no sense in adding wires and pins for boiler flameout or boiler blowdown on an NW-2 switcher, for example. A switcher was not likely to have dynamic brakes, so those wires weren't included, either.

There were adaptor cables, such as 27 pin on one end and 21 pins on the other. I recently built up, rebuilt actually, a 12 to 27 pin cable, for a special application. Today, with the 27 pin standard cable, there is room for growth/options as not all circuits are used. It is a good idea to run all wires end to end on a locomotive, however. This will let a unit without dynamics, as an example, to run between units with DB.

Ages ago, Trains Magazine had an article called 'Lash 'em Up' which covered MU in some detail. (I do take exception to the mis-information about the reversing toggle switch, however. Doesn't work that way-).

EDM

[Allen Hazen]

I think the article EDM mentions is the one I learned what little I know from. I will see tomorrow if I still have it and post a reference if I do.

I think EDM makes an essential point: when the first diesels with m.u. were designed, their pin arrangement was chosen on the basis of what was needed for that application, and only later (as diesels proliferated and many railroads found it useful to m.u. units of different types together) was it realized that a uniform standard would be better, even if it did lead to unused pins on some units. (Why use the same socket on a switcher with no dynamic brakes or steam generator as on units with both? Well, in ten years time someone will want to m.u. the switcher with a roadswitcher ....) What surprises me is that British Rail, which (aside from switchers) had only a few, largely experimental, diesel locomotives until the late 1950s, didn't look at the American experience and demand a common standard on all its road diesels.