it's a pretty simple design, and easy to make (though if you make the track long like mine it can use up a good amount of pieces).
mine is 4 FT. 3 in., but it could be made lots longer.
it is made up of a two long lines of yellow connectors connected with green rods. it has supports every 4 yellow connectors. and front of it has two sets of gears(one hooked up to a motor) that the chain is looped around to send it in the other direction. the back is a almost exact copy of the front. the gears are in the same place, just there is no motor.
well, i hope you like it!
a very similar one is posted here:
Step 1: Car
the top has a rod and connector that hooks up to the chain. it is made so it can turn and move in all directions so it can move when it reaches the turns on the ends of the tracks.
i used a black gray rod in this because it would not work well with a bent rod. black rods are more straight then regular grey rods because they are more resistant to bending.
the pictures are pretty self explanitory.
Step 2: Front Track and Chain Guide
the chain goes around the read gears.
the random connectors on the red rod are just "filler connectors" as in there just there to take the place of spacers and make sure the rod does not slip out.
Step 3: Back Track and Chain Guide.
Step 4: Track Pieces
Step 5: Chain and Finishing Up
now, putting on the chain and connecting the track is all self explanitory so i won't go into that.
to add the car just put it on the track and connect the red connector to the chain.
now turn on the motor and watch the car go back and forth. :-)