Don't know what car was involved, but it happened about the time I got on the Green Line at North Station to go downtown to run a quick errand. I didn't get the errand done (no big deal), but I did get to walk through the tunnel a couple hundred feet from my stranded train to Haymarket Station when the Fire Department ordered the traction power shut down!
Not that it was a particularly pleasant experience - the tunnel was very poorly lit and the benchwalls we were walking along were littered with debris and tripping hazards. I shoulda faked a sprained ankle...