> they would have to do it at a station, where they could immediately get off the train. I think, though, that this would be risky, given that subway stops generally have a lot of people getting on/off the train in the first place. I've seen a phone jacking in this exact scenario and nobody moved to stop the guy running. Nobody on the train can help cause the doors have closed, and nobody on the platform has any idea…
I wonder what you could usefully do with a Kensington lock on the train. I bought one for use in cafés although I haven't used it most of the time. You could attach it to something bulkier or something that you could put under the seat, maybe. I don't remember if New York subway seats have an exposed bar underneath that you could lock it to. I'm sure locking it to the vertical poles in the center of the car would be…
Although I'd highly recommend putting some cloth around it, or fitting it through the belt loops of jeans/trousers to soften the inevitable 'yank' when it comes.