Remixed from the 12-ton coal wagon, I scaled it up to fit the Bachmann Largescale Thomas the tank engine set we run around our christmas tree every year. It's as big as I could fit onto my Malyan M150 printer. 190mmx100mmx45mm. I also fitted the hubs with 623ZZ bearings. The axles call for 3mm, however I used 1/8 piano wire, drilled out the wheels, and ground down the ends slightly to fit the 3mm ID of the bearings. The length is about 77mm for each axle. The hubs, buffer bars, and box are held together with #2 screws at 3/8" length. The couplers are held on with #4 screws at 1/2" length. The brakes are heat welded to the hub.
Coupler is available here: http://www.thingiverse.com/thing:1974185
Print the Box and hubs with 50% infill. Print the brakes, buffer bars, and small parts with 100% infill. Supports are only required for the brakes.
Grind down the 1/8 piano wire ends to fit the 3mm ID of the bearings.