On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled Minecart Mischief. You can begin this quest in the Caustic Caverns location by picking up the Old ECHO item.
1) Old ECHO
2) Mine cart
3) Locked gate
5) Shortcut to Fast Travel
On the pole you will find the Old ECHO , picking up which will activate this mission. Head to the mine cart  through the bridge from the above screenshot. Stand behind the cart at the end of the bridge and start pushing it.
As you approach the locked gate , you will be attacked by a group of Volatile Crystalisks. Get rid of them and open the gate with the lever on its left side.
Keep pushing the cart while opening the locked gates until you reach the storeroom . Push the cart down onto the belt, Use the lever from the above screenshot and prepare for a fight with Varkidams. Once the cart gets crushed, you will be able to pick up five Raw Eridiums. At the spot where you picked up the Eridium you will also be able to end the mission.
In the room you can find a few weapon chests which are worth searching. In order to save up some time, head to the passage shown in the screenshot  which can serve you as a shortcut (the gate at the end can be opened with a valve on its left side).