Exactly what the name suggests, a train, powere by pistons! it can be built at any lenght and will push blocks forward.

If you want to use this on your map/server please read these notes first!

*BUILDING IT
The schematic contains 3 parts: the departure part (with the button). A rail section that can be copied into infinity (in the middle). And the arrival part (optional, you only need an unmovable block to stop the train).
You can also build extra blocks and redstone torches on top of it, to get a different design.

*USING IT
place sticky pistons and normal pistons, facing towards eachother at the very beginning of the track. The normal pistons should touch the iron blocks in the beginning. Place a few solid blocks in front of those and you've got yourself a train! You could place the train somewhere in the middle of the track, but it might not take off properly.
Do not try to ride it, you will fall through. Just stand in front of it like on a real train.

Additional Notes

Also check out the 2.0 http://www.planetminecraft.com/project/piston-monorail-20/
and 3.0 version http://www.planetminecraft.com/project/piston-monorail-30/