Won’t it be unsafe to have high voltage lines all over the place?

No more dangerous than any existing power line. The wires which power the trains will be over twenty feet in their air, well above the tracks. The poles which hold the wires up do not have and sort of ladders or handholds. The wires and supports will be inspected regularly to ensure reliable train operation. And remember that since nobody should be walking along railroad tracks anyway, Caltrain electrification wires will be more removed from people than most street power wiring.