has anyone else had any problems with the Intermountain FT (B) units derailing when going thru crossovers[?]
I’m not familiar with Intermountain FT B units, may give you some help advice here hopefuly. I would check to make sure the trucks swivel properly and that the wheels are guaged correctly. If the units are coupled with couplers, make sure there is enough coupler swing to handle the crossover, same applies if a draw bar is used. Also check that the trackage is in guage and smooth, no humps or kinks. Also might check that electrical power is not interupted causeing the lead unit to go dead and the B unit trying to push it on through. Could be any number of things, you’ll just have to keep looking.
You may try running the B separately(assuming its powered) and also turn the unit around and see if you still have the problem.
I own the FT A & B for six years both powered, no problem.