As a behavior, Minecart is a custom block effect available in the Workshop. When applied, it makes no change to the custom block. However, when a minecart custom block is placed on top of a functioning Factory, the Factory will produce a Minecart doodad. (see below.)
As a doodad, the Minecart high-polygon doodad behaves almost the same as a normal doodad. However, it ascends more slowly, and is not affected by Jump Pads. To enter a Minecart doodad, press X when near it. To get out, jump with the A button.