okay. lets say you have a train that's an amtrak. it uses a f40ph. the session will recognize the lead loco as an f40ph. you wanted to change it to an p42 so you changed the config file. when you run the session, the session will think the f40ph is leading even though the p42 is leading. because of that the session will not recognize the train and it will disappear. that goes the same with helper locomotives and rolling stock in the train. if their changed then the session will only recognize up to where the change has been made. you see what i mean?