Tube train acceleration is 1.3 metres per second. Multiply that by the number of seconds it takes to leave the platform, which is about 14 seconds. 14x1.3=18.2 m/s, which is 40 mph. The line he was on is not a slow line.
Anyway, even if it was less than 40, its still preposterous!
The value of 1.3 m/s^2 (= 1.3 meters per second squared) is a Tube train's *maximum* acceleration; so the train may have been accelerating at a lower rate. However, the speed difference would probably indeed still have been massive. (If the train accelerated at only half of that, 0.65 m/s^2, then the train would still have reached a speed of 30 mph.)
Of course the movie The Tuxedo (2002) already revealed the secret to Bond's extraordinary physical capabilities and skills. :)
What makes you assume that the train is going anywhere near full speed when he jumps on it? It would make sense for them not to travel at full speed while in the station and wait until they are completely out of it to ramp it up, so to speak.
And yet idiots think that the Daniel Craig movies are "more realistic" than the silly old Roger Moore ones. Skyfall has without a doubt the least plausible plot of any Bond film I've ever seen. It's absolutely impossible, but don't tell that to the Dark Knight fanboys who have ruined cinema.
It's been a long time since the series has had a good villain. The villains are really what makes these films good. Sanchez or whatever his name is was just a vanilla, watered down version of 006, and he was so generic that he could have been a villain in any other film series. I could totally see Batman, Jason Bourne, the MCU characters, or Indiana Jones fighting against this villain. He's just some random baddie who does some stuff. Nothing about him resembles a classic Bond villain.