Another Dual Rail Video Camera Dolly / Slider on the market. The wheeled track dolly rides on 42" - 63" metal rails, has adjustable legs, and can support both Bowl Mount fluid heads or standard 3/8" mounted Fluid Heads. The end clamps support tripod mounting, and the dolly can be used both on and off the tracks. Starting around $198 dollars for the 42" version, and can be found via eBay (Click Here).
Unlike camera dollies that use an entire square platform, here's a build from AgeOfRockets.com that uses a configuration of simple planks in a very sturdy frame design. If built correctly, the unit should be able to break down fairly small and portable (less the rails). Full DIY and parts list following the link: http://www.ageofrockets.com/blog/?p=131
Of course there's already a design on the market that shows how it's all put together, and how it's all taken apart. The Swift Dolly (coming out of India), comes with a portable 12' Rail system, rails ends, and portable camera dolly that can be broken down into several parts for traveling. The kit even comes with bags. For the price (and the ugly colored wheels), you might want to look into the DIY version first.
Swift Video Camera Portable Dolly with 12' Track
The hardest part about DIY track dollies are getting the wheels drilled in properly. If your angle of drilling does not have the wheels sitting in proper alignment, you'll get wobbly footage as you push down the track. For those who don't want to tackle the hardest part of cutting and drilling angle iron, there are some ready made dolly wheels that just need to be bolted in place. Just grab a single board, and you're ready to roll...