The original Sperry rail flaw detector cars were two units - a power car and a trailer with the detection gear. But, I think (dangerous, but I'll try to find a good reference with a car number) that this set was owned by the Association of American Railroads. The AAR, in the early 1930's, was developing their own method for rail flaw detection. The AAR used residual magentism, and Sperry used (uses) a heavy electric current (plus an ultrasonic system for the past 45 years or so).
I think the CB&Q might have bought one of the AAR magnetic car sets. The NYC had their own induction car, the X-8015, which carried Sperry equipment. It was a single unit gas-electric car.
