When we moved in 2006 to Roosevelt Island on April 18th the Tram went out of service so we did not step foot in the elevator at the Manhattan tram station until service returned in late August. Once we did we were momentarily perplexed as to what the "B" stood for on the control panel. As we never thought to ask it was some time before we learned that "B" was for Boarding.
If you travel with a child ask them to guess what the "B" stands for. I am guessing you will get a few interesting responses if they don't know the answer.