I have put figures in a couple of MTH engines. It depends on how much room is in the cab of the SW1500. In my case, I purchased the MTH 30-11064 8-Piece Locomotive Cab Figure Set. It includes 4 engineers and 4 chairs that they sit on.
Not really difficult, but as with any shell removal, it's certainly possible to screw things up if you aren't careful. The SW1500 has spring contacts, so there should be no plugs to deal with, just make sure you don't pull wires apart or pinch them putting the shell back on.
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