Terry, check first to see which excursions the cruise line is offering for your specific cruise. You may not have all options available to you if your time in the port is a bit short. As f-mattox said, it's only one train, but there are several ways you can take it: one-way, with a bus return; roundtrip on the train, or combined with a longer excursion into Canada, also often including gold panning and other activities. If your specific sailing offers all of these, then you can opt for any of them, depending on your interest. You will take a slightly different route back by bus, less scenic I believe; if you choose roundtrip on the train, you'll see the same scenery but from a different direction (which can make it look very different), but you will just go to the top of the Pass, turn around and come back. If the activities involved with the longer excursion are of interest to you, and the cruise you are on offers this one, then go for it. (If it is not offered, you may not be in port long enough to take it.)
It's a spectacular ride, whichever of the options you choose - I do recommend it.