UPRR's former SP tracks are north of the field and the proposal is to depress the tracks to allow access to airport property farther north. There is a recent precedent for this at Charlotte Douglas Airport in North Carolina where a new NS intermodal terminal was installed below grade between Runways 18C and 18R:
CLT Taxi Diagram
Two jet bridges cross the new yard to connect a new runway to the west to the remainder of the airport. The NS yard is blind on the south end and connects to an east-west main line north of the airport. Here is additional information about CLT
Charlottefive
Charlotte Business Journal