Previously in San Francisco, back-door boarding was allowed only on Muni's light-rail lines and on certain bus routes if a fare inspector was on hand to collect the fares or check passes and transfers.
the program extends to all railcars and buses, which have been
outfitted with equipment that allows riders to swipe their prepaid
Clipper cards at any door.
(TRIMET LIGHT YEARS AWAY FROM THIS TECHNOLOGY)
Muni's systemwide all-door boarding catches on - SFGate