The Saboteur Walkthrough

Talk to Bishop, and then find Skylar near the Arc de Triomphe. This is a big mission. First, Skylar goes with you to sabotage a bridge. You need to find the four points (highlighted with red lights) to plant sabotage. You're not using dynamite this time. You'll see little ghosts of the sabotage at the exact points to plant the charges. You'll have to climb to reach some of them.

Doing this part disguised is kind of pointless, as there aren't that many enemies, and it's easier just to shoot your way out. When all four charges are planted, take the stairs up to the tracks and plant the detonator on the tracks. Now run back to Skylar.

We're not done yet. After you meet back up with Wilcox, you get the second part of this mission. Slipping onto the train is easy enough, as is planting the initial charge, but things get a bit crazy once the ride starts.

This is more or less a run and gun mission. You can generally stop between cars and recover, before advancing. There is some time pressure, but not much so don't rush it. Recover, take out the enemies ahead, and keep moving.

When you reach the engineer car, flip the switch, and then head back to the prison car to end this mission.