• M.U. Compatibility in early EMDs (TR series switchers in particular)

  • Discussion of Electro-Motive locomotive products and technology, past and present. Official web site can be found here: http://www.emdiesels.com/.
Discussion of Electro-Motive locomotive products and technology, past and present. Official web site can be found here: http://www.emdiesels.com/.

Moderator: GOLDEN-ARM

  by SouthSideDIY
Hey Group

I haven't been here in a long time. I did have an account here when I was in HS...I'm 32 now lol. At any rate, this question has been bugging me.

Did EMD TR series cow-calf switchers utilize the same 27 pin MU system that is still in use today. I am having a hard time finding solid information and photographs. It seems like most original TR-A and especially TR-B hunits have long since either been scrapped or heavily modified. For example, the set at the Boone and Scenic Valley RR isn't quite original. That 'herd' was paired together by the CNW in service, but the 'cow' was a regular NW2 that was built as a single unit. The calf is real, but I imagine it was rebuilt or modified to work with the NW2.

The Belt's TR2s and TR4s are likely the reason for my fixation on these units. I wish I would have gotten up to see that pair that ILSX owned that worked in the upper midwest together for some years. I think one of the A units is still running around on the EL&S. I have seen pictures of these and do see all the air hoses that you would find on newer switchers and road locomotives. I didn't see actual 27 pin plugs though.

I know these were likely modified to an extent by the BRC too. I do find the BRC's choice in 12-567 powered switchers through the years to be interesting. They seem to have started with drawbar connected TR2s, moved on to TR4s(which I think had regular couplings as built). Then they ordered 2 pairs of regular SW9s in 1951 which had MU plugs(no air hoses) on the cab ends only so that they could run in pairs cab to cab. Finally they just ordered regular SW1200s with full MU (hoses and plug/wires) on both ends. They must have realized that the cost savings of cabless b-units and units with MU plugs only on one end wasn't worth the reduced versatility. Maybe it was a union thing.

When all of these units were in service in the latter half of the 20th century, the BRC never seems to have mixed them up in service. They should have all been compatible. I just haven't seen any photographic evidence. The closest I have seen was a calf coupled in front of one of the SW9s at the shop in Clearing(that SW9 had no MU on the front, so we know they weren't paired). Every other picture of them working has been of them in matched pairs. The only exception I can think of is the SW1200 that was turned into a slug and run with an SW1500.

The SW9s were compatible with other locomotives. Like the TR4s in Minnesota, pairs of the SW9s would famously run in pairs in secondhand shortline service in the early-mid 2000s. I do know of one instance where one of the SW9s was run with a rebuilt IC SW14(rebuilt from an SW9). There is even a video of it on youtube. I was told that they worked "ok" together. I believe the SW14 had regular 62:15 road gearing, so it would push the BRC unit around during low speed maneuvers. I have read similar anecdotes about mixing higher geared EMDs(Fast 40s, F40PHs, various GP40 based commuter locos, and passenger FP9s in freight service).

At any rate, can someone tell me how the calves were controlled as built? I would imagine by the time the 70s and 80s rolled around, there were enough scrapped locomotives around to easily retrofit more standard MU controls if they did not already have them.

Oh pps: I seem to recall there being compatibility issues with MU on early E and F units, like the ones originally built with drawbars.
  by BR&P
Not all locos used 27-pin, had an SW9 which I believe had fewer pins. Never paid attention because we didn't have anything to MU it WITH. But be aware the units you are interested in may have had something other than 27-pin.

I have an EMD Operating Manual #2313, for SW8 and SW9, and for TR5 and TR6. Second edition, September 1951. A VERY brief look doesn't show a mention of MU cables but it may be in there. If that book interests you we can talk.
  by Pneudyne
The Trains 1968 December article “Lash “em Up” is useful reference. I have attached a scan of it in this and following postings. parts.
Trains 196812 p.44,45.jpg
Trains 196812 p.46.jpg
Trains 196812 p.47.jpg
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  by Pneudyne
Trains 196812 p.48.jpg
Trains 196812 p.49.jpg
Trains 196812 p.50.jpg
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  by Pneudyne
As far as I know, but not necessarily correct, EMD made the change to 27-pin MU jumpers with the F3 in the F-series and with the E-8 in the E-series.

The later FT (and I think the F-2) had 21-pin jumpers, although the earlier FT appears to have had 17-pm. That needs verification, though.

The E-7 (and probably the preceding models in that series) had 16-pin jumpers.

As to the MU-fitted switchers, I do not know. But the change to 27- pin might have happened at the start of 1948, which does seem to have been a breakpoint.

The EMD operating manual for the SW-1 and NW-1, 4th edition, 1949 June included the following comment:


Governor, Governor Speed, Safety Control Hydraulic and Electro-Hydraulic Controlled Governors)

The engines on locomotives delivered after Jan. 1, 1948, are equipped with a Woodward Governor which includes a pneumatic-hydraulic governor speed control, or an electro-hydraulic control on units equipped for multiple unit operation.


I am not sure whether the Woodward governor would have been the “New SI”, or the PG. The latter appeared in the late 1940s, but I do not know exactly when As far as I know, the first use of the “New SI” with integral electro-hydraulic speed control was on the F-3, which also introduced the 27-pin MU convention. Hitherto on road units EMD had used the “old” SI, with external speed control via an electropneumatic unit. This was also used on the E-7, and probably on the SW-1 and NW-1, when MU-equipped, for pre-1948 production. The E-8 went over to internal speed control (possibly with the PG governor from the start) and to the 27-pin MU protocol. Pre-1948 non-MU switchers had the “old” SI governor with mechanical speed control.

From that, it is a reasonable assumption, but not proven, that the MU switchers from 1948 would have had the 27-pin MU protocol. Before that, perhaps 16-pin as on the E-7 or 17 or 21-pin as on the FT? But there could have been customization for individual customers.

The cow-and-calf transfer units I think would have followed the same pattern as the MU-equipped switchers, with a divide at the beginning of 1948. But whether the control interconnection between the two parts followed MU practice, or whether it was an ad hoc cable is unknown. The latter seems possible given that a fixed bar coupling was usually used.

The move to a standard MU system was probably driven mostly by the railroads. The builders probably did not have much incentive to do this amongst themselves (and may even have been wary of antitrust considerations), but fulfilling customer requirements would have been justification for action. Then at some stage it was taken up by the AAR. Harmonization of dynamic braking control was the last hurdle. The railroads at times also devised their own solutions to MU compatibility problems somewhat independently of the builders.

Note though that the throttle protocol for all of the above MU-equipped EMD units was the same, so they were fundamentally compatible. So, it was possible to couple units with different MU jumper pinouts provided that a suitable adapting MU humper cable was used. Or, non-27 pin units could be converted to 27-pin.

Air braking systems were another consideration, in that the various #6 systems (and 14-EL) as used mostly on switchers and road-switchers were not very compatible with 24-RL, as used on some road locomotives. 26L, at least in its Universal form (probably the majority, perhaps the only variant used in North America) solved that problem, as it was compatible with both #6 and 24-RL. (There were other 26L variants though, used overseas, with different compatibilities.)

  by AllenPHazen
A very early instance of the "cow and calf" configuration (end-cab switcher attached to cabless variant) was built for the Illinois Central... using the innards (16 cylinder engine etc) of the FT. So maybe it, at least, used whatever was used n FT units at the time.
(Not that "it makes sense to me as a layman" is a very strong argument in matters of technology!)
  by BR&P
Gotta love it. Guy joins the forum, asks a detailed question, several members provide helpful info - and a month later we have heard nothing more from the OP. :( Oh well.....hopefully we provided the info he was after.