Mineshaft/Structure

Abandoned Mine Shaft
Abandoned mine shafts generate naturally in the Overworld, they are usually found underground but some can be found in the ocean. They are generated before chunk features such as ores. As with other features, multiple abandoned mine shafts can generate near each other and intersect, producing larger complexes. They can also intersect with other cave systems. Corridors will generate a floor of wooden planks if they cross over empty space.

In Pocket Edition, they can spawn on the surface in mesa biomes. These are composed of dark oak planks and fences, rather than oak.

Blueprint
You can place the Minecart with Chest anywhere on top of Rail. The Monster Spawner should contain Cave Spider.