Not suggesting your track was this way but many have a keyhole slot...pull
track, it slides back to the wider hole in the slot, lift track off.

OUCH! Actually, the old rail DID have keyhole slots, but the
contractor (builder) didn't use them, it looked like they used
washer-head screws and a power driver, right through the rail base.
And judging by the condition of the rail (nasty bends in the lip,
paint on it but not on the wood inside the pocket), I bet it was
salvaged.

-Zz