I think this is a good idea to determine demand but I'm guessing they'll find that most people want their bikes with them, not left behind.

For me, parking my bike is never an option because I need it to complete my commute.
CalTrain does a great job of getting me up the road, but the last 2 miles I need my bike. Since my bike is an integral component to my commute, leaving it behind will never happen.

I would bike to Caltrain and leave my bike at the station if I had some confidence that it would not be vandalized or stolen. I take Caltrain to San Francisco where public transit is frequent so I don't need my bike. Many other people travel to stations where shuttle busses can take them to work. Installing day-use bike lockers at Caltrain stations is surely much less expensive than building more car parking lots. I only use Caltrain a couple of times a week, so renting a locker for a full year at a time is wasteful.