I'm fairly sure Burlington Northern never owned a Challenger, so I'll go with freelance I think!
I'm sticking solidly to early BN locos apart from that though, and will be modelling an alpine area with plenty of hills and coniferous forest as far as the eye can see. I'm going to have pine tree making down to a fine art by the time I'm finished, or the folks at Woodland Scenics will be a whole lot richer.
BN was large enough that I'm sure they travelled through such a region, though I've never bothered to check this because it's of no concern to me.
I chose to model Burlington Northern based almost entirely on their colour scheme.