As in other types of traffic accidents, skid marks can be measured and used to determine the speed of the train in cases where the engineer applied the emergency braking system. After the emergency brake is set, it takes approximately 1 second for every 19 cars (1,000 square feet) for the system to engage fully. To give the metal wheels grabbing power during braking, sand is released when the emergency brake is applied. The investigator should ascertain whether the emergency brake was applied and if the manual sander is in working order and not out of sand. At most crossings, a whistle board alerts the engineer of the approaching crossing. Often the engineer will remember where the train was in relation to the board at the time the brake was applied. The automatic brake valve handle and its position also should be examined to determine if the emergency brake was engaged. Other factors to consider include the condition of the rails, the weight of the train, the amount of the grade, and the type of brake material (cast iron shoes or high friction composition shoes). Most locomotives are equipped with a speed recorder and a device that also preserves data on such devises as the headlight, reverser, and throttle. These data can be indirectly used to determine traction motor current plus distance traveled, elapsed time, and air brake operation.