The phrase "header rail" refers to a horizontal bar or beam that is located at the top of something (like a window or door) to provide support and structure. Full definition
It's totally automatic, unlike the soft - top, which has to be unlatched from the windscreen header rail before you fold it away — and can be raised or lowered when the car is moving at up to 6mph. (carbuyer.co.uk)
For # 41.14 my hood was restitched, its plastic header rail fixtures were replaced and its internal wiring re-tensioned. (evo.co.uk)
But with a thick header rail, shallow windscreen, wide centre console and narrow dimensions it's always going to be very snug in here. (evo.co.uk)