Originally Posted by
BeantownFlyer
My experience is you can pick it up anytime up to the scheduled departure time of the train, but if the train is delayed, and you try to pick it up after the scheduled departure time you may find your reservation cancelled.
This is also true if the tickets are for an unreserved coach train (i.e. Capitol Corridor) that is good for up to a year after issue date. So if you miss the original ticketed train your ticket is cancelled and you must purchase a walkup ticket if you want to catch a train later the same day.
Moral of the story: Get your ticket early if you even think you might miss the train ticketed.