I´ve recently built this track dolly and it was successful, it works well. You can build your own if you like, just watch the tutorials below and the tests and you´re good to go.
I´ve shot a short film with this dolly as well, check it out HERE.
Packable, fits in a suitcase (without the tracks obviously)
Inlineskate wheels, softer and smoother motion than with skateboard wheels
Bent edges for stronger profiles and steadier tripod in some cases.
Adjustable wheel tightness and distance between wheels
An extension for “low angle fly cam” shots
All holes and bolts are the same size so it basically works like lego.