turning locomotives around at terminals

I have a naive question. If a trainset coming into a terminal is not in a push-pull configuration,
where/how does the locomotive get switched to be the front of the outgoing train?

Specifically:
I know Grand Central Terminal had loop arounds, but I don't think they are in use anymore.

And what about Washington DC's Union Station, which seems to be a true Terminal configuration?

thanks...

Typically, the trains were backed out of the terminal, or pulled out by a terminal switcher, to a nearby yard or engine terminal where the power would be switched out. In steam days, many of these yards or engine terminals had a turntable or, space permitting, a wye. One practice that the P-RSL had was to install "escape switches" to a parallel track so that the locomotive could be uncoupled and switched to the adjacent track without needing to move the entire train.

Washington Union Station has a wye in the yard/engine service area. For turning passenger cars (obs cars), it had (has still?) a turntable in the general vicinity of the platforms, in the area where the tracks between the platforms narrowed down to the throat of the tracks.

Washington Union Station diagram showing turntable:
http://broadway.pennsyrr.com/Rail/Prr/M ... term_k.gif" onclick="window.open(this.href);return false;
http://www.washingtonterminal.org/12-wa ... urntable-2" onclick="window.open(this.href);return false;
http://memory.loc.gov/pnp/habshaer/dc/d ... 0166pv.jpg" onclick="window.open(this.href);return false;

I believe Memphis Union Station also had a turntable