Bulkhead flatcars are designed with sturdy end-walls (bulkheads) to prevent loads from shifting past the ends of the car. Loads typically carried are pipe, steel slabs, utility poles and lumber, though lumber and utility poles are increasingly being hauled by skeleton cars. Bulkheads are typically lightweight when empty. An empty bulkhead on a train puts it at a speed restriction to go no more than 50 mph (80 km/h). Since bulkheads are lightweight when empty, hunting can occur when the car is above 50 mph (80 km/h). Hunting is the wobbling movement of the trucks on a freight car or a locomotive. If the wheels hunt against the rails for a period of time, there is a high risk of a derailment.