Steve, MTH did not get involved with this engine. Their loss.
You could get MTH Dorfan passenger cars to pull with it, but again, the green does not match.
I use my green 3930 T Reproductions engine to pull the set of vintage Dorfan freight. These are worth picking up as you come across them. They sell surprisingly reasonably, considering how uncommon they are. Fun to collect one at a time. I got replacement MTH Dorfan trucks from Midge to put under them, since they usually come with disintegrated trucks or Lionel replacements - which I guess is one thing that keeps their price down.
The Dorfan freight cars, even in C-5 or C-6, are really quite something, and almost always better in person than photos. They are the only Standard Gauge lithographed freight. This engine goes quite nicely with them.
It's a great little engine, runs flawlessly. Glad you got one, excellent b-day present!