Employees were connecting railcars in preparation for later use. One of the employees was standing in-between two sets of railcars. The employee was trying to straighten out the coupler on one of the cars that was attached to the engine so that it could connect to the string of stationary railcars. A second set of railcars (approximately 10) rolled forward and struck/pinned the employee's arm between the two couplers. The second set of railcars that rolled forward did not have a chock installed under the wheel. The employee suffered fractures, severe bruising to the arm, and compartment syndrome, requiring hospitalization and surgery.