The Sheriff of Lynchwood sends a train through the area with Eridium and Brick wants you to blow it up. You can get to the demolition depository by heading into the little valley with the Skags and taking the elevator that goes upwards. Clear the area if needed and find the switch that is marked on your map. Using this will start the five-minute timer and you’ll need to quickly make your way to the next switch to close a door.

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Clear the area of enemies first (left). Quickly press the switch to close the door before the car arrives (right).

This will stop the cart with the bomb in it from leaving the area, so once it stops, grab it and start moving towards the next objective. Remember that the timer is still going, so it might be wise to ignore enemies and head straight for the waypoint. Drop the bomb car off, then hurry to the detonator. You’ll need to wait until the train is on top of the bomb car before pushing the detonator, which makes the train go kaboom, finishing the mission.