Around Sacramento, at least, UP seems to use the lead locomotive road and number as the train address. Needs the road (UP, BNSF, Amtrak, etc.) due to trackage rights and connections. Occasionally, some kind of train ID is also used - such as the Amtrak train number, or the "Folsom Turn" which switches much more than just the remains of the old Folsom-Placerville line. In track warrant territory, such as beyond CTC limits on the Roseville Sub, authority is nearly always issued to trains based on locomotive numbers not train IDs, even for Amtrak. Check out railroadradio.net if you want to hear it in action and don't live nearby with a scanner.