When you arrive at Union Station, you can take the BNSF train directly to Downers Grove Main Street, then either take a cab or arrange for a shuttle with the Westin. Their website says that hotel shuttle service is available within a five-mile radius. You could also walk from Union Station to the Ogilvie Transportation Center where you could take the Union Pacific West train to Lombard. I think the Main Street station in Downers Grove and the Lombard station are about the same distance to the Westin.

metrarail.com/metra/…schedule.full.html