Didn’t the DMU haul more than one Bombardier commuter car when it was testing on Miami’s Tri-Rail?
If I recall correctly, the limitation of keeping engines running on RDCs under tow is related to oil, all right… but it’s oil lubricating the bearings in the transmission, not oil in the torque-converter circuit causing hydrodynamic braking. This would be similar to the reason you’re not supposed to tow most automatic-transmission vehicles with the drive wheels on the ground for long distances.
Meanwhile, of course you could keep those transmission temps down by the same method adrianspeeder probably knows well (if he’s familiar with strokin’ loads as well as git’n-er-dun quickly): nice big external hydraulic-fluid coolers, probably with their own set of cooling fans. (Show me a motor-electric car, btw, that doesn’t have forced cooling for appropriate parts of its “transmission”: traction-motor blowers.) Of course, what’s the point of doing this for a couple of 270-hp truck engines already pulling a fairly big carbody?
For sheer fun, though, why discuss RDCs, when there are probably New Yorkers on this list who will remember SUBWAY CARS switching freight… ;-O
Overmod – I’m pretty sure you’re right on the bearings. My recollection – which is hazy at best at this distance – is that the pump supplying oil under pressure to the transmission bearings and so on on those Allisons was driven by the input shaft, not the output shaft, so the input shaft had to be spinning to get oil pressure – which meant the engine had to be running.
I do not recall any subway cars switching freight, but a number of the 900-series and 1000 series open-platform wood Brooklyn United elevated cars were converted into work motors with both trolley poles and third rail shoes and did substitute for the usual steeple cab locomotives in South Brooklyn on-the-street, right of way, and elevated freight service, including trackage now used by New York and Atlantic and New York Dock railway, all of which at one time had trolley wire overhead. Some steel subway cars, including “Red Birds” have been converted into work motors and do haul hopper cars and gondolas for track maintenance and rebuilding, looking exacly like a subway car pulled freight train. Earlier steel cars were also used in this kind of service, and the IND division even had some specialized cars that looked like subway cars with windows and some doors missing. In addition to ex elevated cars, the South Brooklyn sometimes would borrow a streecar system crane car or snowplow for use as a freight locomotive, and I occasionally bumbed a ride! Lots of adapter couplers around and zero attempt to use the freight car airbrakes. Generally only up to three or four cars would be handled at one time. No train longer than a city block was allowed anyway.