There are lots of enemies waiting for you in the sewers including Tushkans and Snorks. After arriving at the hall in the middle of the map [#2], you will be attacked by Monolith soldiers. Additionally some zombies will appear at the exit. It's worth to search the cars (and trains) along the way, you will find lots of ammo and medicine, sometimes weapons or even a SEVA suit.
You begin (0) at the lowest part of the map [#1]. Dogs (kill them with a machinegun) and a Snork (rifle) will attack you around the first intersection (1). You will meet some more dogs near the gate (screen). Press the button (2) and prepare your rifle to fight two Snorks.
(3) is a place with an anomaly - you can pass it using the prototype detector in hand or by following your comrades, as they should find a proper path on their own. At the end of the tunnel (4), some more dogs will appear. You have to press the button and enter the corridor on the left. Two little dogs and another button (5). A narrow corridor leading north contains two dogs and one Snork that can attack you from behind. Move to map [#2]. There are three Snorks (screen) by the entrance (6).
3 more will attack you a little further (7). The button (8) will open the gate leading to a big hangar with dogs inside. A group of Snorks will attack you as the gate is opening, luckily they're weaker than the normal ones and a couple good shoots should be enough to get rid of them. You can an artifact from the small room on the right (9). To the east of (10) there's an anomaly which you can avoid with the detector. At (11) there's an entrance to a side room (screen). Inside, in a chest (orange point) there's a SEVA suit.
Go up the ladder (12), save the game and press the button on the northern panel. At this moment, heavily armed enemies will come out of the small room (9). If you have a machine gun with an optical sight or a sniper rifle, it's a great moment to use it. In case any of your companions die, you can always load the game. After the fight go down, carefully cross the anomaly and enter the building (10). Once you get to the upper floor, to the place the minimap shows, save your game and open the door. Snipers will appear on the right and left side of the main hall (screen). You can wait for your companions to take them down. A couple second later a group of enemy soldiers will enter through the corridor (13). You have to activate the button showed by the minimap.
You can continue your journey through the north, narrow corridor (14). There may be enemies waiting for you by the entrance. There's a chemical anomaly at (15). Move carefully with the detector in your hand. Use the button (16) to open the road to the part of the sewers with a big amount of zombies. Look out for dogs and 2 Snorks by (17). (18) is a zombie terrain, just like (19). At (20) you will find a long ladder that will end your journey.