Main purpose for the 3rd track, as I understand it, is to allow passenger trains to bypass the freight trains waiting in the vicinity of Roseville. UP won't have to clear things out at passenger train times on the other main tracks and yard leads to let Amtrak by. And when the passenger trains aren't there - at 10 trains/day or so they won't be around all that often - it's a good bypass for through trains that don't really have to spend time in the yard. Read the DEIR for more information. Think of what you could do as a yardmaster if you could get those pesky passenger trains out of your traffic pattern.