Actually it's a set of 4 New Haven silver cars. The Railway Post Office and 2 others are powered and the fourth is a dummy. I'm glad it's just rubber bands. I'll replace them all.The terminology you use is not familiar to
many of us here on the Forum.
Could this be the 'car' that you refer to?
I obtained a beautiful, very old model of BART (the San Francisco Bay Area's electric commuter train) from Athearn. The powered car uses a motor with rubber drive belts (rubber bands connecting the drive shafts to the axles). To put it bluntly, this old train barely runs. I'm not too thrilled...www.modeltrainforum.com
If not, post a photo of what you have?