Free MVgo commuter shuttle is available from Mountain View Caltrain to the Shoreline area on weekdays only. These shuttles drop off about 10 minute walk from the Amphitheatre (get off near Shoreline and Charleston intersection, where Rhythm Shuttle picks up at the end of the show, and walk the rest of the way).
Two of the three shuttle routes serve the Shoreline area (Green/West Bayshore and Orange/East Bayshore).
Expect delays due to weekday traffic in the area. Real time shuttle tracking is available.