Originally Posted by RogerD408

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 latter the same day.

Moral of the story: Get your ticket early if you even think you might miss the train ticketed.