This walkthrough is meant as a general guide only. It may not include every pickup or secret. There may also be alternative ways of approaching situations within the game.
The level begins with Lara in a long concrete room. Head forward, climb the crate on the right and pickup the Grenade Gun ammo. Continue through the corridor, hug the right wall as two sentry guns materialize in the next room. Run to the right, following the right wall behind the crates in the south/east corner of the room. Push the floor lever, climb the orange patterned crate ahead and the higher crate on the left. Immediately crouch down at the top to avoid the gunfire and watch a camera shot of a gate opening in the previous room. Still crouched, pickup the Crossbow ammo and drop back down onto the orange patterned crate. Face north, drop down and shoot the gas can in the west corridor (to clear the way for later on). Climb back over the orange crate and follow the left wall back into the previous concrete room. Head to the crawlspace in the south/west corner. Jump up and grab the edge, pull up while holding the crouch and "up arrow" keys simultaneously (so that Lara doesn't bump her head and jump back down). Crawl through the ductwork, lower into the next room and arm weapons. Drop onto the yellow block below and shoot the two SAS guards. Pickup the Crossbow and Shotgun ammo and climb back up. Jump up and grab the edge of the crawlspace, pull up while holding the crouch and "up arrow" keys simultaneously. Crawl through the ductwork and lower back into the concrete room. Head back to the sentry gun room and run into the entrance in the west wall (hug the left to avoid being torched by the sentry gun).
Head west, past the stairs to the opening in the west wall and safety drop down to the train wreck below (minimal loss of life). Light a flare, jump back and slide part way down the hole, grabbing the edge. Shimmy left around the corner until you reach the ladder on the inner north wall and climb down. Crawl through the ductwork until you can stand, arm weapons and shoot the two bats.
[NOTE: The following may be a secret but is a necessary step to proceed through the game]
Secret #1: Face west and jump up and grab the black textured wall (it's climbable). Climb up into the alcove above into Secret #1 and shoot the two bats inside. Push the floor lever and watch a camera shot of a room and door opening in a previous area.
Climb the ladder to the top of the shaft, pull up and crawl through some more ductwork. Lower down into the hole at the end, climb down the ladder on the inner wall and drop onto the train below. Light a flare and jump up and grab the jump switch on the south wall. Head north and pickup the Grenade Gun. Drop to the ground on the left and pickup the Grenade Gun ammo. Head south down the corridor and pickup the Revolver ammo at the end. Climb the series of blocks to the top, head through the corridor and pickup the large medipack. Push the floor lever in the next room and pickup the small medipack, Uzis and Shotgun ammo and exit. Continue north into the room at the other end, climb the pop machine and pickup the Lasersight. Exit, turn left and head up the stairs on the left. Arm weapons, shoot the two SAS guards and continue up the stairs into a new room with three sloped blocks.
Take a running jump to the first sloped block, jump off the first, jump off the second and land on the third. [Note: If you accidentally fall to the ground floor, jump up and grab the sloped east wall near the first sloped block. Pull up, backflip to the first sloped block, jump off the first, jump off the second and land on the third.] Arm weapons and take out the SAS guard on the south ledge. Take a running jump and grab to the south ledge (or used the climbable east wall). Push the floor lever and use the monkey swing above to get to the north ledge. Push the floor lever and head through the entrance to the next area.
Walk onto the first grated tile which is slightly lighter in colour and safe. Angle right to face south/east and take a running jump to the next lighter coloured tile ahead. Angle left to face north/east and take a running jump to the lighter tile across the gap. While still facing north/east, take a running jump with a curve left to the next light tile. Angle to face south/east and take a running jump to the lighter tile in front of the exit in the east wall. (see screenshot sequence) Exit the room.
[NOTE: I was unable to find a way to grab the pickups on the fire tiles without torching to death. But I suspect the builder is teasing us, as the same pickups are available in the next area.]
Slide down the slope as far down as possible and jump and grab the block ahead. Slide down the next slope, into a pool of water. Swim left and pickup the small medipack, Shotgun ammo, Shotgun, large medipack and Grenade Gun ammo around the bottom of the pool. Swim to the opening in the middle structure (north part of the pool), pull up, arm weapons and take out the three SAS guards inside. Push the floor lever and pickup the large medipack in the north/west corner of the room. Exit, swim right and enter the underwater maze.
Follow the right wall, ignore the pickup for now, swim until you can swim up through a hole in the ceiling and surface for air. Pull up into the small room, push the floor lever and watch a camera shot of an underwater door opening (takes a while). [Note: The underwater door actually opens as you step near the floor lever, but I pushed the floor lever anyway.] Pickup the large medipack and Uzi ammo in the south/west corner and drop back into the water. Swim east, following the right wall and pickup the large medipack you saw before. Swim straight forward to the end until you can swim up into another small room. Pickup the Uzi and Crossbow ammo and drop back into the water. Follow the right wall around until you can swim up through a hole in the ceiling and surface for air. Pull up into the small room, arm weapons and shoot the two SAS guards. Push the floor lever and watch a camera shot of a trap door opening in an area further on. Pickup the Revolver in the north/east corner and drop back into the water. Swim west, follow the left wall until you can swim up through the opened door and surface. [Note: There are a few extra pickups in the underwater maze (another Shotgun, ammo and small medipack) but felt was not worth backtracking for.]
Head through the west entrance into an area with semi-sloped columns. Angle left and take a running jump and grab to the column on the left. Take a standing jump to the column ahead and pickup the Revolver ammo. Take a running jump to the next column, turn right and another running jump to the next column. Pickup the large medipack and take a standing jump south, back to the previous column. Take a running jump and grab west to the next column and a running jump to the ledge at the end. Turn right and head to the hole in the north corner and climb down the ladder. Jump and grab the jumpswitch and climb back up. Head through the entrance, arm weapons and take out the two SAS guards in the hallway. Enter the room and shoot another two SAS guards inside. Climb the grey platform and the yellow block and pickup the coveted Nitrous Oxide Feeder. Jump up and grab the monkey swing above, as the grey platform is now on fire. Swing east, follow the ductwork with the "extra rivets", turn left and head to the north wall. Let go and grab the ladder, climb down and grab the jumpswitch near the bottom. Exit the room, head right and climb the ladder inside the doorway in the south wall. Head down the corridor and safety drop through the newly opened trap door (minimal loss of life) back to the Sentry Gun Room.
Combine the Revolver and Lasersight and take a step down to the lower crate marked "explosive" (not too far as there are mines). Take out the two sentry guns by aiming for the red and white canister at the back. Pickup yet another Shotgun and Shotgun ammo and return to the unmarked crate behind. Climb the upper "explosive" crate, hug the wall and safety drop to the floor. Continue south along the wall, stop at the door and turn left. Head forward (east) to the far wall and turn left. Head to the corner, climb the explosive crate and drop to the other side. Head through the door, slide down the slope and this enjoyable level ends.. very well done for this builder's first level.