Complicated but much simpler. Can test the longest steam PRR S1, A-A, A-B and even an A-A Centipede Lionel and MTH. On backside can test rolling stock functions couplers, operating cars etc.
Can even test these after had JAK make a modification for me.
Just select the correct support rail sections for make and length and go.
Edit: Oh can test at flank speed without loco wobbling off the rollers or the loco kicking the rollers away.