Start by vacuuming the tracks thoroughly, using a small vacuum cleaner brush so you can clean all those hard to reach spots.
Run a damp washing cloth over the whole surface.
Wrap the cloth around the tip of a small knife or blunt tool (a skewer would do too) and clean or loosen up those hard to reach spots that still have dirt on them
Pour vinegar in a spray bottle and spray window tracks lightly.
Sprinkle baking soda, making sure to cover all the corners and other hard to reach spots. The baking soda will bubble and help loosen up all the grime. If you feel like you need to apply more vinegar, do so.
Leave on for a couple of minutes then wipe with a damp washing cloth to remove most of the baking soda and vinegar paste.
Grab your q-tips and start scrubbing the window tracks in the most hard to reach spots. Then use the q-tips to remove the baking soda paste. I found it useful to have the q-tips wet.
You may need to use the knife/blunt tool again for those nasty corners.
Wipe everything down with the damp cloth once more then leave to dry and give it a last quick vacuum to remove any baking soda traces that are stuck to the window tracks. Make sure the brush is clean!