2005.01.02: Xmobile:

The Xmobile: a true piston powered vehicle in Halflife2; it is powered entirely by the force of the explosions on the pistons.

Tips:
-Keep re-pressing the strafe keys to keep turning.
-There is a 'viscous' effect that pushes the Xmobile forward when the clutch is disengaged and the engine is in forward or reverse.
You can use this to your advantage sometimes.
-Try using the grav gun on the rear wheels or pistons if the Xmobile needs a bit of help.
-To control the vehicle, 'use' one of the prisoner pods; there is one stationary, and one connected to the top of the Xmoblie.
-There are a couple of buttons around that you can press if you want to see the entities working.
-Noclip comes in handy.

Issues:
-It's very hard to walk around the decks of the vehicle while it's in motion. :(
-Reverse doesn't work very well.
-Steering while backing down a hill doesn't work very well.
-I hope your computer has cojones.

Download Xmobile BSP + VMF