I'm going to not answer the question you are asking, but instead offer an alternative excursion recommendation (that still involves the train). I'm not trying to be annoying or contrary, so if you don't want this unsolicited bit of advice, move along.
Still here? Awesome!!! I would highly recommend doing a bus up/train down or train up/bus down excursion. The views are completely different and with the bus, you get to stop and really see the sights and explore a bit. I would have been really bummed to be on the train the whole time.
We booked privately with Chilkoot Charters (due to recommendations on the DIS here). We were picked up at 8:15 on a bus, taken to numerous sights along the way by an incredible tour guide and then we enjoyed lunch at a summer dog sled camp (where there was an option to ride for an additional charge - which we did). Then we were taken to the train and enjoyed the ride on the way back down where we saw completely different sights (and some of the same but from a different vantage point) which we loved. We arrived back mid-afternoon, explored the town and did a little shopping before racing back to the ship just before it started raining and a cold wind blew in.