First, go and make a wall. Second, make a hole in the wall. Now, where you made a hole in the wall at, place a block down, along with a railroad track on top of it, and then a minecart with chest. Now, break the block under the railroad. And, the minecart shall falldown. And, if the minecart is sideways, and inside the wall. You shall, now cover up the wall with any block. And, now you can open up the chest in the wall, by looking in the corner of the two blocks.

Video Tutorial: