We thought it was very worth it do to the White Pass railway....it is a bit on the long side, I think around 3 hours and you come back the same route as going up, but the scenery alone is worth seeing and its very imformative on the way up. I doozed off coming back along with others, just something about the train ride.
It's a great excursion. They have you swap sides of the train you're sitting on the way then down... but on the way up you're better off to be sitting near the windows because that's when most of the narration is done. So that is on the left hand side of the train. It's very beautiful.