The recent P2K RS-2 is a sweet locomotive. The Atlas Trainmaster is also very nice. The Atlas/Kato RS-3 is nice, if not 100% accurate — they certainly run well. The new Atlas Century series are very nice. Stewart has some very nice models of early Baldwin switchers. There are good covered wagons from Proto, Stewart, InterMountain, and others.
There have been problems with cracked gears in earlier runs of Proto GP7 and GP9s, something to do with forcing an Imperial gear on to a metric axle. This is not that big a deal — if they break, replace them with Athearn or NWSL items. I have several of them, have had no problems to date, and like them lots. That said, I know one day the problem may surface, and I will have to fix them.
I suggest you establish your time frame, find a roster, and start choosing which locomotives you want to run. Then ask about specific locomotives, and you will get specific answers.
Cheers