Electrification has been in Caltrain's (and SP's before it as noted elsewhere) plans for ages. But no money to do it. Now HSR (if it doesn't all go down the toilet) is supposed to pay for it, and electric is needed to run in a tunnel to the new Transbay Terminal anyway (also needs HSR funding for the tunnel). So stay tuned - it might or might not happen, and when is a very open question. Caltrain has clearly shown, though, that with high and rising diesel prices if it doesn't happen they will have to shut down at some point in the near future.