This project is a cute 4 studs wide cargotrain. It's made up of a steam engine, tender, cargo trailer, and refrigerated truck. It runs on an oval of track which has a sloped section on either side. The higher part of the track is supported by arches. I've tested it thoroughly and it's great fun to play with, runs very smoothly, and never accidentally comes off of the track.
I built this project as I wanted to have a Lego train layout with an oval of track but I don't really have enough spare room for the usual 6 studs wide Lego trains.
So far Lego have never made a train for 4 studs wide train track (only a couple of roller-coasters) and I think that this would be a great new product that would appeal to all ages.