This is a C-15 50 foot baggage car that is unique to the B&O. It has nothing to do with any Presidential train and was announced by Sunset at the same time as the Army hospital car.
The B&O had 100 of these cars numbered 1500-1599 (hence C-15) and they were seen all over the system - mostly if not always used for sealed transport of mail and express between cities. They did not have end doors, and I would venture that they never carried an automobile. For passenger trains that offered baggage service that would have been done with either a combine or a 70 foot baggage car.
The B&O did have some horse express cars with end doors and probably FDR's car was carried in one of those.