Though it seems a like a lot of risk. The device would be spendy to make, and if ti falls off or the authorities snag it, that's a fairly good sized investment lost. And really, if you saw someone sticking one of these to the side of the train, how many of you wouldn't think "Hell no, I'm not getting on that train! Theres a bomb stuck to the side!!!"
But if the technology works, how long till marketting dweebs are paying the subways of the work to install these and prject advertisements onto the walls of the subways?