The Mine Shaft "Calamo-T Mine":

We went all out in the back this year building a 10x20 foot haunted mine shaft.

The mine was a 10x20 foot frame of steel conduit (we bought out every piece of 1" conduit from every local Home Depot and Lowes) with fittings and (fire retardant) tarps we bought from tarps.com - highly recommended!). All the tarps were black on the inside and olive-drab on the outside. The outside edges of the mineshaft/tent were mounted onto 6x6 landscaping timbers for stability. Here's the frame being set up:

On the inside I used scrap lumber tie-wrapped to the poles to both hide the poles and give the look of rickety wooden supports for the mine shaft:

The interior was pimped out with some creatures, some skulls, some LED accent lights (part of a zip line of LED lights that lit up the whole graveyard - from Minionsweb.com) and finally a set of Cowlacious.com 's "cave eyes" were mounted along the ceiling in the first hallway - giving a great creepy effect of 32 separate sets of "eyes" slowly blinking on and off randomly.

Some fabric scraps, an old lantern, some vultures, and some more scrap lumber completed the facade:

To complete the look inside, an old miner (static) prop was created and seated at the end of the first long passage way:

And at the exit, this creepy guy hung out:

For the big surprise inside the mine shaft,  a runaway mine cart complete with skeleton passenger would jump out at passer-bys.

The cart needed to be light - and would only be seen from the front so I opted to basically make it a facade - a front and two partial sides of very lightweight lauan plywood painted black and glued and screwed to a chunk of 2x8. An old tuna can painted rust brown with a couple LEDs mounted inside and a red-plastic cover taped over it makes a primitive headlight for the cart.

For the rails I used a 10' piece of steel channel, cut in half (2 5' sections) painted rust-brown and attached to scrap 1x4 lumber as railroad-ties.

To give the cart motion but make it as stable as possible it has two cart wheels mounted on the underside of the 2x8 which lineup with the slotted channel on the track.
Also attached to the bottom of the 2x8 is a 2x3 that extends back about 18". The 2x3 serves as both a mount for the cylinder that propels the cart forward as well as a guide to keep it lined up. The "guide" 2x3 sits on top of another 2x3 which is screwed down onto the rails; on top of this bottom 2x3 I screwed down l-shaped metal flashing - 2 pieces screwed down onto the 2x3 so it forms a channel inside - i.e.  |_ and _| . At the end of the channel I screwed in a steel  L-bracket with a hole cut out  for the cylinder end to screw into (see first photo below). To help the top 2x3 slide easily up and down this channel, I mounted a pair of sliding closet door wheels on the end of the 2x3 (see the second picture below). After it was all assembled I painted the assembly black so it wouldn't be visible (or reflect light) in the mine shaft.

Finally it was time for the controls. As with almost all my props, the brains is a EFX-TEK PROP1 controller which I programmed specifically for this prop. You can see how I mounted the controls in the photos below.

The PROP1 drives a solenoid (mounted bottom right as seen in the photos below) which controls the cylinder that launches the cart. A couple variable exhaust ports on the solenoid allowed us to fine tune the cylinder in both directions - giving a satisfying (but non-destructive) crash as it moves forward, and a much gentler return as it resets. The PROP1 also controls a pair or LED lights; one is the head lamp on the cart itself; the other is a couple white LEDs mounted in a small spike which I positioned about 2 feet in front of the cart (hidden along the floor) to illuminate the cart driver when the cart lurches forward. A PIR motion detector is also hooked up and mounted just under the hand of the cart driver - detecting victims as they approach. To really finish things off, we drive a Rogue Robotics uMP3 player for sound. Wild man Will mixed us up a custom mix of water-dripping, low moans, crying, and all around creepy background noise that I setup to run continuously until the prop gets triggered. 

Once triggered, after a very short delay I light the LEDs and interrupt the background music and kick in the sound track of an imminent train crash - squeaking wheels and a crashing sound that announce the cart slamming forward. Below are a couple shots of the final setup of the cart in the mine shaft. A section of chicken wire was mounted in front of the cart to keep anyone from getting too close (or hit) by the cart. The cart enclave was positioned just past the end of the first passage in the mine shaft - just as you came around the corner.

Finally, here's a short video where we were first testing the cart in the workshop (before final timing) and a short video of the final prop live in action on Halloween night.