How do you get past the subway trap on batman arkham city

You have to have played the game far enough so that you've obtained the Line Launcher, and you also need to have upgraded it so you can stand on the line.
When you finish hacking the console in the subway deathtrap, fire the line launcher and stand on the line. Just wait there, the electricity will come on below you, and eventually go off, so you can gather the trophies and exit safely.