The term "top rail" refers to a horizontal bar or beam located at the highest point of something, like a fence or a railing. It is the uppermost part that you can hold or lean on for support. Full definition
Spray - on liner covers entire bed interior surface below side rails, including tailgate, front box top rail, gauge hole plugs and lower tie down loops. (paulmiller.com)
Leave the outside / top rail loose, with the edge isolated in the rings. (zolowear.com)
We love its geometric steel tube design, wooden top rail, and soft matte finish. (modernnursery.com)