I think the big problem is not that the land will be used for something other than what it currently is, the problem is WHAT it will be used for if UP puts the yard there. Fans of the park are probably most concerned with the noise and diesel exhaust deteriorating their favorite park.
If I had to guess as to why UP wants so much land, it is so they can later sell the excess to developers for a profit, or, they are concerned that as the area developes, the new home owners will raise a big stink about being so close to a rail yard and try to force them out, kind of like Denver did when they closed Stapleton Airport and build DIA. There was nothing wrong with Stapleton, 35% unoccupied, but the homeowners complained that it was lowering their property value, despite the fact that the airport was there long before they purchased their homes. UP might be trying to keep a large buffer between the switching yard and future homes.