If your watching Season 2, Be sure to watch the Extras,
There is a vignette about building their second locomotive, which unlike the first locomotive (which was made on the quick almost entirely out of wood and was deteriorating rapidly), is now almost entirely made out of steel, and will last for several more seasons.
This new locomotive has a very cut down pick-up truck chassis integrated inside the tender body to move the trains along, the old loco needed a trackmobile to push the trains along.