On a steam locomotive, the backhead is the part of the boiler that pushes into the cab. A number of controls are mounted on the backhead for use by the engineer and other train personnel, and it can also have a firebox door to allow the crew to check on the fire and add fuel, if necessary. In regions where steam trains still run, passengers may be able to see this part of the train if they can get close to the cab; it will be easy to identify because of all the controls. Museums can also provide an opportunity to see the boiler and backhead assembly up close.
Operators on a steam locomotive need to be able to control the conditions in the boiler to determine the train's speed and other operating conditions. One important control on the backhead is the throttle, connected to a valve to control steam pressure. People can close or open the valve to force the train to speed up or slow down. Pressure gauges providing feedback about the steam pressure are also an important component. The train needs to build up pressure before it can start running, a process known as “building up a head of steam,” and the operator needs to maintain pressure within a safe range to keep the train going and prevent boiler explosions.
Depending on the brake system, brake controls as well as brake pressure gauges can also be found in this part of the cab. To monitor water levels in the boiler, operators use a sight glass as well as check valves. It is important to keep the water level stable to prevent accidents and keep the boiler in good working condition, and a busy locomotive can go through water very quickly. The bell cord and train whistle controls are also typically mounted on the backhead.
The firebox door, usually located near the floor of the cab, provides an opportunity to look into the firebox, add fuel to stoke it, and move the fuel around if it is not performing as desired. The train operators need tools for manipulating fuel as well as grasping the door safely, as it can become very hot when the boiler is in operation.
The backhead can cause the cab to get quite hot, depending on the insulation level of the boiler. Ventilation may be provided through an open cab design or by windows that people can adjust to allow more or less air to flow through the cab. The proximity to the boiler can also make the cab dangerous, as it is virtually impossible to survive a boiler explosion at close range.
