After the long train ride, you will drop off just before the large underground section. You will then need to proceed on foot to sneak across or fight through a battlefield.
For the stealth route, head forward and keep to the left. Walk down the stairs, and head to the left track. You should overhear soldiers to the right talking about a secret passage. Wait for the conversation to finish, and for the officer to depart. Once he leaves, you can sneak past the other guard, hiding on the right-hand side of the pillar before jumping over the rail.
After getting across, head to the left for the first walkway, walk across the pipes until you reach a horizontal beam, and walk across it to the right-hand wall. Follow the arrow pointing up, and the arrow pointing across. You can find a gasmask if you have a cracked one, but the rest of the trip will be below in the poisonous fumes. Climb down onto the pipe and reach the bottom.
As you approach the manhole, look for tripwires. If you step on one, either run back, or bolt forward as fast as you can. When you reach the manhole, look for a ladder on the right. Climb two ladders, and you are now behind enemy lines.
The first enemy room has three Nazi guards. Wait for one to approach from the side passage before heading to the spiral staircase. When you reach the top, look for broken glass at the corner just before the doorway to the main tunnel. The main tunnel is patrolled by two guards, as well as a trolley that moves back and forth, but there is enough cover to avoid detection.
At the first Nazi camp, hide behind the sandbag wall at the top of the stairs on the right of the tracks. Wait for the trolley to approach the front line, and you can sneak into the tunnel behind the camp. Enter the rooms to the left, collect any ammo, and open the final door to start the next section.