I would recommend leaving the team in the start area. Start moving forward (CAREFULLY!!). You will have to take care of a security camera (#1). It's been located on your left. You should know what has to be done by now. Once you've dealt with this problem gather the whole squad around you. Keep moving forward. You should be able to see a few terrorists in the distance (#2). If you have a scope, you might want to consider taking them out right now.
Make sure you have cleared the area. Enter the main cave with your team in order to deal with the remaining terrorists. After that, head on to a small door (#1). It's locked, so you will have to spend some time working on it. Open the door. Be careful! The next room is being occupied by AT LEAST three additional units. Start off by killing a guy who's standing on your left (#2).
Remaining terrorists will probably use smoke grenades in order to blind you and your team members. I would recommend sending the whole squad inside (#1). You could deal with the terrorist threat on your own, however this would be very risky. Keep moving forward. You will find yourself in an engine room. Choose the only available passageway (#2). I would recommend leaving the rest of the squad right here.
Zoom in the view and start eliminating visible enemy soldiers (#1). I have to warn you, though. The area is being guarded by large groups of terrorists. Don't even try attacking all of them at the same time. Step up a little bit closer. You should notice a security camera on your left. Get back to the junction. Head on to the LEFT path. Now you will have to get closer to the camera. Make sure you're not spotted (#2)!! Neutralize this object. The rest of the team may join you now.
Enter the main building of the underground complex. The door is located beneath the camera. Once you're there, stop by the destroyed wall. You will have an opportunity to make a surprise attack against two enemy soldiers (#1). Leave the rest of the team near the destroyed wall. Go back to the camera. Turn left. You will be encountering some massive forces along the way (#2), mainly because the terrorists were planning on attacking your team by surprise.
Keep heading towards the main hall. Remember that some of the terrorists are coming this way (#1)!! Start moving towards the carriage. You should be able to spot some new enemies on the other side of it (#2). Eliminate them. Once they're gone, gather the team around your position.
Use the small stairs in order to get to the other side of the tracks. You will have to neutralize the remaining terrorists (#1). Thankfully, they're not that hard to find. After that, you will receive a short message. It turns out that some of the terrorists have escaped to a nearby subway station. You will have to hunt them down. Keep heading towards a marked spot. You will find a door here (#2). Stand close to it. The game will load the next map in a few seconds.