In light of the London bombings, I've been thinking it might be safest to ride in the front cars of subway trains. In addition to being at the head of the tunnel for an easy escape, more importantly, a terrorist may seek the middle of the train for the most destruction, especially when passing another train. Perhaps being in front would blow the front car the furthest away from the rest of the damage.
But what about a collision? It would be worse to be in the very front or back carriage in that scenario. I believe passengers in one of the Tube trains had to walk a couple of miles up the line to the next station, even though the train had just left the previous stop, as they could not get through the train and the tunnels are no narrow.
If I remember correctly, either the front or back carriage on the Tube is the one gay men use for cruising :-D