It will not fit under a 40' box. By the time you put drive shafts and a universal on it, you have no room left for a standard Atlas or Tomytec motor. The 50' is enough of a squeeze.
I did a test of putting a Bachmann 44-tonner chassis under a 4-axle USRA tender (Rivarossi, Kato) and it worked. Just sayin'. That's one of the best robust short 8x8 drives out there.