Get the train early in the morning, along with every other traveller in Riobamba and Ecuadorians as well. Find yourself a spot on the roof of the train amongst rucksacks, chickens and everyone else, and you're off! It is safe enough on the roof, and although not too comfortable the scenery takes your mind off it. Passing the large, snow capped volcanoes such as Cotopaxi on the way and passing down the mainstreets of Andean villages makes it amazing even before getting to the Devil's Nose. Here you pass on the very edge of the mountain side, expect a very large drop on one side, but it is really breathtaking.
My advice is to take lots of layers of clothing as it is freezing in the morning and boiling by the time you get to the Devil's Nose. Take some food with you.