Well actually it wasn't for free, if you've been with Trainz long enough you'd remember that the original BigBoy was a payware model. Auran later made arrangements with its creator to include it in TRS2004. I wouldn't exactly call it freeware now either as the only way you can only get it is by buying a copy of the game from Auran - not dl free from DLS or some 3rd party site.
By today's standards it's a bit dated but was and still is quite impressive.
Bob Pearson