Rain Water enters through tracks of sliding glass windows

The lower track of a window frame normally is supposed to have small holes at its lower edge toward the outside that allows water in the track to drain out. These are commonly called weep holes.

It is a common problem that over years dirt can collect on the bottom of the track and clog up these holes so the track will no longer drain. Look to see if yours are clogged and clean if necessary.

The tracks and their weep-holes in the picture look perfect. So, I'd suspect that there's a crack in the tracks or that you need to re-caulk the bottom of the window unit on the outside or that the sashes need new seals.

If a metal unit, you can cheat the sash seals by pinching the tracks slightly closed with pliers to re-tighten the tracks to the sash seals. If that works, then you'd want to repeat it on the entire window to restore heating or cooling efficiency to the place.

The tracks are usually, and seem to be here as well, a single piece. But, they're commonly poorly joined miters at the corners. You'll need to caulk the corners, if the water's coming in at the window ends.