These track are designed to be used to create or breakdown trains of carts. They must be powered by redstone to function. Right clicking on a coupler track will change it to decoupler mode and vice versa.
Coupler Tracks can be chained to linkup an entire Train when combined with a Train Detector.
Coupler tracks have an additional mode called Auto-Coupling which can be configured with a crowbar. This sets a cart to couple with the next cart it collides with.