@Gerry Those are nice engines - can you thin something else? I had an absolutely beautiful MTH Premier Southern Crescent engine (PS3) and 5 car passenger set. Took me about 18 months to build the set. After a while I realized I did not have a connection to that railroad and decided to sell the set off. I broke even on the deals. As I have learned the O scale hobby, I have become more interested in the scale size (high rail) part of the hobby, so I have been selling off my RailKing engines, all of which are articulated engines. (I like BIG steam). The only "little steam" I have is a Premier GS4 Daylight - loved that engine since I was a kid - and she's not all that little.
Since the engines still fit what you are modeling, I would hang onto them for a later time. It will only cost more to replace them, if you can find them. That said, if you decide to sell the Challenger, I would probably be interested in buying it from you. Best ... Jeff