Princess suggests at least one hour between tours (click http://www.princess.com/learn/faq_an...excursions.jsp
and select "multiple excursions in one port"). Keep in mind that the duration of the first tour is approximate. You definitely do not want to miss the White Pass RR -- yet, you might have a bit more of a cushion there as the train leaves a little later than the meeting time, although there will be some cutoff point for checking in. You can check the train schedule as it gets closer.