The venerable Dana Dolly - don't arrive on set with out it. 
16 Custom Dolly Track Wheels
Soft Polyurethane Track Wheels
ABEC-7 Rated Precision Bearings
Wheel Trucks Swivel for Curved Track
For 1.5 to 1.9" O.D. Pipe
Supports 100 lb of Camera & Accessories
Center Support
1/4"-20 & 3/8"16 Accessory Holes
Includes 75, 100 & 150mm Bowl Adapters