[EnderViaAnsible]

He's also a double non-sequiter in most games he's modded into. Not only are there no trains, there are no trains with faces.

He stands out instantly as out of place in a way that some other in-jokes might not. Hank Hill might not look super weird in Left 4 Dead; presumably a number of middle-aged Southern men with an avid interest in propane were caught in the zombie apocalypse.

Because of this, it's funny even if you don't know who Thomas the Tank is. I think this is part of the reason for Shrek memes persisting as well (in addition to Shrek's extremely dedicated fanbase).