I think Stringy has it right. It's enuff to drive you nuts, but I'll take a crack at this:TRAX
carts are made by Wheeleez for Hobie and have the scupper "Plugin" design made to fit Hobie hulls, but are OK for many other brands as well. (I use them with Feel Free kayak models). STANDARD HOBIE CART HOBIE TRAX CART
Came in a narrow
and a wide model
Each had slightly different Twist-lock PVC tires. I own both types. The wheels are interchangeable.
The wide version came with post collars. There is no "cart keeper" pin on the narrow model. HOBIE TRAX "2" CART
Larger, polyurethane twist-lock tires. Cart rated at 176lbs. Still features a 1/2" stainless steel solid axle.HOBIE HD CART
Designed for the TI. Foam filled rubber tires rated at 200lbs per. Tubular 1/2" axle, with 2 sets of holes for lynch pins. Does not take twist-lock inflatable tires.HOBIE HD CONVERSION KIT
Fits standard cart only and allows you to upgrade to the HD wheels. Not compatible with the TRAX carts. Except with Stringy's clever mods. http://www.hobiecat.com/forums/viewtopic.php?f=69&t=21856
If I have this right, the TRAX models have a solid axle w/ notched end
The HD model has a tubular (hollow) axle requiring pins.
And I now have a headache,..