Is there any way to track a specific router interface and change the default route accordingly ?
Either use Netwatch on a local interface IP, or use the check-gateway property of the route itself.
http://wiki.mikrotik.com/wiki/Manual:Tools/Netwatch
http://wiki.mikrotik.com/wiki/Manual:IP/Route#General_properties
Or watch interface events in the logs via scheduled scripts, but that’s sort of messy.