The Texas Eagles used to back in most times. At one time they were trying to keep the Engines away from the Station for polution reasons, hence the backing into the Platform area.The City of New Orleans (and the corridor trains to/from Champaign & Carbondale) will always back in because there is no forward move connection to Union Station.
In the past a lot of trains would back in when they carried freight at the rear, for some operational reason that I never fully gathered. Also, when they did more through-running between east and west (such as Capitol Limited/Southwest Chief) they would back in. Otherwise, it's mostly straight in.
Beyond that, they will almost always pull straight in