Well, choosing an average Subway Car, with a weight of 82,000 lbs, seating 54. Fill it with all seats taken and that number again standing gives just short of 100,000 lbs per car. Put eight cars together and you get 800,000 lbs.

Now, run that car at 60 miles per hour, or roughly 90 ft per second. Give spidey three minutes (180 seconds) to stop the train. He needs to slow the train at 0.5 ft/s^2. Force = mass * accelleration, so that's 400,000 lb ft/s^2.

Now, gravity is 32 ft/s^2. So the force required to stop the train is the same as 12,500 lbs under gravity, or the same force as lifting 5.6 tons. However, that's the average force. Since lastic-type compounds exert force roughly linearly to their stretch, which means that Spidey's webbing would start off exerting almost no force, but to achieve an average of 5.6 tons, they would need to be exerting 11 tons at the end of those two minutes.

The tearing strength of a limb is typically greater than it's lifting strength. I'm told that a human leg will tear off somewhere around 1,000 kg. That is a lot more than most people can leg-press, probably four times pressing strength. If Spidey can chest-press 10 tons (Marvel Handbook figure), then you'd have to figure him good for 40 tons before he lost an arm.

As for the webbing. It's accepted that Spidey isn't strong enough to tear his own webbing, so 11 tons should be fine there, especially since he laid on a pile of webbing. The only real question might be whether or not Spidey could exert enough hand-grip strength to hold the webbing as shown in the film? But then again, why didn't he just stick the webbing to the train, like he stuck the other end to the walls? Heck, I don't know. I'm just a physicist.