On a window a railing is the horizontal portion of the frame at the top and bottom of the sash. Rails contain grooves that hold the window panes. Muntins extend from rail to rail.
Chris Hill
The term “rail” is not exclusive to windows — it’s used widely with furniture to reference horizontal parts.
Mathews Brothers Company
Window locks can be put on the railing.
calarchitecturaltraditions.com
Muntins extend up and down between rails.