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).
Dual Rail Dolly Track Video Camera Slider