Race Track Lab is a free browser based slot track designer.

Being web based, Race Track Lab requires no download or installation, and can be used on any device; desktop, laptop, tablet or phone. It supports Scalextric (SCX), Ninco, Carrera, Policar, and Scaleauto tracks, with Scalextric (UK), 1/43, and HO to come.
The system provides real-scale measurements, multi-lane layouts, inventory management and a community circuit library.
I’ve checked out Race Track Lab myself, and it seems well designed, intuitive, and simple to use.
Race Track Lab
UPDATE:- Race Track Lab now supports Scalextric (UK) Sport track, and now has the option to toggle from metric to imperial sizes.