How come no baby train?

Back then, my father took a ride on the Union Pacific, and he overheard this conversation between a mother and her little girl:

— Mommy, mommy, how come the big cow in the field has a baby cow with her, the big horse in the field has a baby horse, but the big train doesn’t have a baby train?

— I dunno, sweetheart. I guess you should ask the conductor. Well, here he comes!

The conductor comes around, and as he goes by the little girl’s seat, she asks him:

— Mister conductor, how come the big cow in the field has a baby cow with her, the big horse in the field has a baby horse, but the big train doesn’t have a baby train?

— My little girl, that’s because Union Pacific always pulls out on time!