A vertical fall arrest safety device travels along a rail of a vertical track and is configured for quick attachment and detachment anywhere along the rail. The device has a mounting body having side rail engagements configured for slidably engaging behind the edges of a rail of the vertical track in use. The engagements are spaced and staggered so as to receive the rail therebetween when the mounting body is at an angle, and then slidably attach to the rail when the mounting body is twisted into alignment with the rail. The device also has a lever pivotally coupled to the mounting body at a fulcrum. As such, when a downward force is applied to a connection point of a distal end of the lever, an opposite distal end of the lever frictionally engages the rail.