A grind rail is an object used by skateboarders to do skateboarding trick

Skateboarding trick

A skateboarding trick, or simply a trick is a maneuver performed on a skateboard while skateboarding. Learning and perfecting new tricks is the primary goal of many skateboarders, for whom most of the time spent skateboarding is spent on tricks....

s on such as grind

Grind (sport)

Grinding is a sliding stunt performed in many extreme sports. Stances in which slides are performed are known as grinds.Grinding is performed by sliding across the top of an obstacle, and can be accomplished in a number of ways...

s 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.