This is the second car in the train that Line Mar shows on their box. In the 1930s, when this set was sold, steam locomotives were the standard. Such locomotives carried their fuel supply behind them. The coal tender was often referred to as just the tender, as it is in this instruction sheet.
The tender was a really simple build. And it was also one where reality was at odds with the fantasy illustration. The dowels holding the bumpers are shown resting on the axels, nicely parallel to the bottom of the tender. They don't quite line up that way, causing the dowels to droop down a little.