If I buy an Amtrak ticket from say Pontiac, MI to Chicago, can I get on the train at Detroit (three short stations down) if I happen to be closer to Detroit at departure time?
Technically, this is not allowed. The Amtrak Terms of Transportation states:
If you do not board your train as booked, your entire reservation is subject to cancellation.
However, you can call Amtrak and try to change your reservation from Pontiac to Detroit. You may even get a partial refund if the fare would be less.
For the best answer to that question, call 1-800-USA-RAIL (800-872-7245).
Second best, http://www.amtrak.com/contact-us. (but do us all a favor, and don’t ask “Julie“)