mtalin, that's good to know about the T--perhaps there's been a clarification or change in policy since that incident.
ChainlessRev, I take my folder on board the NYC subway all the time, including rush hours. I once tried to take a folder onto a full bus and they let me on board, but it was extremely awkward lugging it up the steps, and letting people go by (bus aisles are narrow!). They really need to put front racks on the buses.