@Maersk.146 @classick Thanks! It's probably because this line was single-track first and second mainline was built later, but with different alignment. This may be because different gradient or curve radius were needed.
This train is actually headed away from Barstow. It is westbound at Frost (Victorville) and headed toward San Bernardino. This flyover at Frost was built because the train is about to enter Cajon Pass, which has left-hand running for part of the pass, due to the different grades on the various uphill and downhill lines, which are on separated alignments.