Apparently stuff like this happens a lot, and these perverts rely on the usual reaction of the victim (yes, you were a victim), to be too shocked to make a scene so that they get away with it. If it happens again, or for anyone else this may happen to, bear this in mind. Instead of thinking 'OMG if I say anything people are going to think xyz', shout your head off, preferably mentioning what the offender is doing, e.g. 'Hey you, the fat bald guy with his penis out masturbating whilst watching me, how dare you?' etc. to embarrass him. Make sure you get witnesses, don't waste time having doubts before you raise the alarm for others because he'll just put it away (or cover it up like in the case of the OP). Then call the police and report him. Don't feel mean about it either, these people are disgusting perverts, sitting on public transport, watching young women and girls whilst masturbating, it's absolutely sick, and it's happening more and more often. Women need to start fighting back. They deserve to be prosecuted for behaving like this (and more), and I'm pretty sure it'll make them think twice about whipping their dick out another time.