How do I climb onto the top of the train?

On the first mission, just after the bad guy plants explosives on the door and decouples the other half of the train, you are left hanging on to rungs at the side of the train by just your fingertips.

At this point the game appears to prompt you to hit the spacebar, which I do, but each time the prompt turns red, indicating that I have failed in some way, and I fall back down to the bottom rung of the ladder. After about three tries you fall off and die.

Is there a knack to the timing of pressing the spacebar (the prompt appears to pulsate) or is there something really obvious I am missing?

You're supposed to tap the spacebar as fast as you can. This is indicated by the input prompt flashing rapidly.

If it turns red, you simply didn't tap fast enough, and the action fails. Fail enough times and you're dead.