Grind rail
A grind rail is an object used by skateboarders to do skateboarding tricks on such as grinds and slides. It is usually square or round.
There are two types of rails:
Handrail, a normal handrail on the sides (or the middle) of a stair
Flatbar, any type of rail you can find in a skatepark (if it's not kinked)
It is also commonly used in freestyle skiing, skiers perform tricks off and onto a rail and also while grinding it. Rails can be all different shapes and sizes, usually ending with a small to medium drop to the landing.
See also
Skateboarding trick