An employee was setting a string of four railcars in the yard with his groundsman. The groundsman secured the ground hand brake on the first railcar and unhooked the string from the railcar. The employee noticed a railcar starting to move as the manual ground brake on the first railcar failed. He attempted to set the brake on the second car when the railcar wheel smashed his steel toe boot to the ground, fracturing his right foot and removing a toenail.