There are several solutions to turning around at a terminal. Sometimes there's a turnaround loop. Grand Central has two, one on each level. Older systems would

There are several solutions to turning around at a terminal. Sometimes there's a turnaround loop. Grand Central has two, one on each level. Older systems would detach the engine, rotate it on a turntable, and reattach it at the other end of the train. Double-ended trains are far more common today, since control from either end was solved long ago.