Also near tracks 12 there should be a desk worker
Some Metra trains arrive at Chicago Union Station. If you're arriving on one of these trains, go through the sliding doors at the end of the platform. Find an Amtrak employee; if you don't see one, go to the Great Hall by turning right if you are arriving from Milwaukee District or North Central trains, or left if you are arriving from BNSF, Heritage Corridor, or Sothwest trains. The boarding process depends on a lot of things, including whether you have kids, are a senior, are military, etc. If you are first class, you can go to the lounge and they'll take care of you there.
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