With real estate in Lower Downtown Denver being a very, very hot commodity now and land valuations there having skyrocketed and continuing to do so, I seriously doubt that UP's valuation of the property is the problem here.
Note that many business in that area, some of which have been there for 100 years, are being forced to sell out and move because land values have risen so much that they can no longer afford the property taxes.
Besides, UP is not the only owner; there are at least 4 parties involved in the DUT ownership...Trillium is, IIRC, the majority owner since it bought out the ownership shares of some the railroads that are no longer with us and some that are but who chose to sell.
The villan here is RTD, which is the government version of a control freak.