Just about everything will run, with the exception of enormous steam big boys possibly. What you'll have to be careful of is extra-long passenger and freight (autoracks, TOFCs etc). They'll need longer truck-mounted couplers to make sure everything stays connected, and most importantly you need to make sure there's clearance between the tracks -- the usual 1 - 1.25 inch won't be enough, it'll clobber stuff going past on the other track...
