I would have to look at the current contract Amtrak has with UP and Metrolink, but the last one I saw stated the the UP Alhambra Subdivison would be the primary route for #1 & #2 and the Metrolink San Gabriel Subdivision was an alternative route. AS the individual said above, however, Metrolink generally won't take #1 and #2 that way, because of scheduled Metrolink trains that #1 & #2 would conflict with, since there is no real good, AVAILABLE passing track/siding to put #1 & #2 in on this stretch to meet Metrolink trains. Also, it should be noted that scheduled, revenue Commuter trains have priority over Amtrak trains, the only trains that have such. After that, according to the Rail Passenger Services Act of 1970, then Amtrak trains have priority over all other train moves, inclduing light engines!