RDCs were new equipment, of fairly restricted potential use, and therefore would have had to be capitalized accordingly. It was from Budd, so expensive stainless expensively fabricated. Meanwhile, commuter service was turning into more and more of a money hole, and in those days there were no government commuter agencies to pick up the tab.
Meanwhile, here came the great die-off of passenger service. All sorts of E units and all kind of rolling stock was available cheap, and could be kept running with minimal expense to serve the mandated clientele.
I have read that a number of railroads were enthusiastic about using the early Alco 2400hp C-C in commuter service... but of course didn't have either the money or the revenue prospect. Lightweight commuter equipment of striking design was developed by GO transit... note: Government of Ontario... and the EL got wonderful new engines and lightweight HEP equipment for them to pull. One of those trains compares handily with the RDC equivalent of at least 16 to 20 separate Detroit 6-110s and all the complicated doodads to make each car self-powered, MU capable, etc.
I don't consider them particularly unsafe, no more than, say, Silverliners. ATSF swore off them after a wreck, but NYC found out that one could go 183mph on the stock trucks...
Most gas-electrics were built to haul trailers if demand warranted. That was emphatically not the case with the first-generation RDCs; I believe Budd went so far as to say any warranty was void if you 'towed' with them... even special lightweight cars. I often wondered whether using 'trailer-towing-grade' transmission coolers for the torque converters would let a couple of cars be towed with AEM-7-style initial acceleration... but I think if a number of railroads that really 'needed' that kind of economy could have done it, they would.
NYC and a couple of other roads did a somewhat GWR-slip-coach-in-reverse thing with RDCs: they coupled them to the rear of regular trains (in the NYC's case, electric MU trains, so that they could be run self-powered to off-main-line destinations or as off-peak service, then coupled in and run without crew expense as coaches. Something I still haven't gotten a full answer on is whether one of the car's diesel engines, which was necessarily running for lighting and air conditioning, was left on down the Park Avenue Tunnel into Grand Central when so towed.
The one place RDCs were 'embraced' for commuter service was Boston, where used cars were actively purchased to increase the 'fleet'. Dave Klepper or another of our members familiar with Beantown operations can tell us what was special about those operations -- I suspect some of the attraction was that no more than two or three cars would be necessary for many services, especially for off-peak services, but something more than an FCD-style railbus (essentially a motorized PCC) would be needed.
