Remove the top of one side of the door casing using a pry bar. Take off the side that exposes the open channel of the track and not the flat/back side of the railing.
Close the 1900s pocket door, then place shims under the pocket door on top of the threshold to make it level and clear the threshold when sliding.
Adjust the railing assembly or jam nut with a wrench until the railing/door is correctly leveled.
Pull the shims out and allow the 1900s pocket door to hang freely. Open and close the door to ensure it hangs plumb from the railing/track.
Replace the top of the door casing using a hammer and finishing nails.