[ckuehl]

BART doesn't have that type of ticket. You tag your Clipper card (or insert your BART ticket, if you're not a frequent rider) upon entering a station, and again when exiting at your destination.

[rosstex]

But CalTrain does have employees occasionally walk through the cars scanning Clipper cards.

[LarryMade2]

Yep and I've seen them catch people who just hopped on.