For those of you that enjoy remote mountain wilderness, put this trip on your bucket list. My wife and I both agreed this was the highlight of our trip. Spencer Glacier is in a remote area, accessed only by train. You're dropped off at Spencer Whistle Stop, and take a short bus ride (or alternate hike) to the lake. There, you'll be rowed out amongst the icebergs, before traveling down river back to the train tracks and your trip back to civilization. The whole trip was amazing - our guide, Dez, did an incredible job. I can't recommend this job highly enough.
The only advice I'd offer is if you're taking the trip out of Girdwood and have a car, you can drive a couple miles down the road to pick up the train in Portage. If you do this, you can shave a lot of transit time off your trip, as the train departs Girdwood, then heads to Whittier for a 40 minute stop. We didn't mind the detour to Whittier, as we were interested in seeing the city anyway, but if you don't want the detour to Whittier, definitely board the train in Portage.