The answer, practically speaking, is that you almost never do. In-service BART cars almost never carry tags — the combination of security, camera coverage, and the agency's quick-response flagging system keeps the fleet cleaner than most urban rail systems its age. Which is part of what made this particular car notable enough to photograph: it was genuinely unusual.

The comments ran toward practicalities. The etching, several people noted, goes into the metal rather than sitting on top of it, which makes removal harder and more expensive than buffing off paint. One commenter mentioned their workplace deals with similar acid-based tags semi-regularly and that the process is neither quick nor cheap. Another predicted — correctly, as these things usually go — that the car would get red-flagged and pulled from rotation fast.

BARTA has not commented publicly on the specific car or its removal timeline.

If you rode BART this past weekend and caught a glimpse of it from the platform — lower panel, metalwork, the kind of piece that takes more than a marker and thirty seconds — you probably won't see it again. The agency's standard practice is removal before the car returns to service, which means the window was always going to be short. What someone walking the platform at Embarcadero or 16th this week will see is the usual: clean sides, the familiar silver and tan, nothing that wasn't there yesterday.