Russ,
That's why BNSF wound up with trackage rights on the Moffat. Grand Jct-based BNSF crews run trains between Denver and Provo. UP crews handle them west to BNSF's former WP rails. It's all about ensuring competition.
The natural flow of the former Santa Fe at Pueblo favors the TP route. I imagine that's why UP is holding on to it. They probably lose less money in property taxes on the dormant line than they would if they lost business to their chief rival via the same corridor were they to sell.
-JDF