When you meet Echo in the sewers, there's a cutscene. Just behind Echo will be a tunnel. Run towards it firing your NanoGL or grenades from your NICW. If you destroy the assault buggy and kill the soldiers fast enough, one of them will drop the Railgun.
After the first level transition after reaching the Citadel, open the door and look to the left. On the ground is a trapdoor. Blast it. Be careful: on your right will be three soldiers; one high and two at street-level. Drop down into the trap door. Activate nightvision. Head down the tunnel. There's a wall to blast. Look at the floor beyond the wall. Destroy the planks. Drop into the hole, squat and move down the tunnel. Pass through the next 'S' tunnel. When you enter the steam chamber, you'll hear soldiers talking. On the platform above will be three soldiers. Kill them with a grenade, or two. Climb up the ladder. As you approach the two W.A.S.P.s on the back wall, you'll get a Bonus Objective award.
When you see Sopot's gunship land, head for the building on the far side of the roof. There will be two guards that come out from behind some machinery. Kill them. Enter the room. There are two guards inside. Kill them. Check the wall behind the stairs. Blast it. Go through the hole and look for a ladder. Climb it. At the top, there will be a W.A.S.P. And 12 reloads. Before you head back down, watch out, there will be a guard at the bottom...
Ok here's a tip that might help you. Alot of times on the game it will seem like there is no way to go. But if you look around there is usually a secret passage or sometimes when there is a poster on the wall that means that if you shoot it it will open up a passage that you can climb through.