John How many UP engines do you work on? The UP stuff has separately attached ladders to the side of the tender. Your not soldering the brass steps to the tender shell, your soldering them back on to the ladder/step assembly, which is separately attached to the tender shell. I think even the IMP RK BB is done that way.
My very first post asked if it was diecast or brass. We know we can't solder diecast. There is a lot of add on brass to MTH Engines.
I have attached a few picture of Premier Challenger, RK Big Boy and last is PR GN Challenger. While the GN has die cast steps all the other detail ladders, and add on is brass. G