There's no need to trench the yard. The line will simply descend east of the yard. Trenching doesn't mean it will be 30' below grade for every inch of the multi-mile segment across the airport, I'm sure they will have a gentle enough descending grade leaving the yard and design around what little on the northwest corner isn't fully below grade (if any) in that transition.

UP may relocate the yard and sell the land at some point on their own as property values increase. Or they and the airport may negotiate a deal to do both as sometimes happens in these types of projects. But moving the yard isn't required to do the trench.