Ah well, your helicopter crashes. You can't hear your teammates, and have to get to where you can hear them.
Work your way along, usually you have no choice about which way to go. Pretty soon you get into the train area. Talk to the vagrants to learn that trains are dangerous. Usually you have enough time to get from niche to niche before the train nails you. There are some military sorts down here that try to interfere with you, and little bug-robots too.
Luckily you're not down there for long. But you're barely out when you're told to take down a bridge. In essence you have to go down in little hatches by three pillars, and blow out each one to get to the inside of the pillar. GeoMod :). Start at once end and work your way across and make sure you get the confirmation at each one that you have successfully made the plant. You should see a little object with lights when you're done with each one.
When you're all set, you get a lovely cut-scene of the bridge blowing up as you jump up out of the sewers.