In this view from the other end of the locomotive, we are looking through the fire door into the firebox with the combustion chamber and rear tube sheet beyond. During the annual inspection, the boiler is “squeezed” above its normal working pressure and checked for leaks. The pressure is then reduced to slightly below working pressure and the staybolts “hammer tested” – a broken bolt makes a different sound when its end is hit with a hammer than does a good bolt. Finally, the steam space is entered and inspected. In the picture you may see some “soap stone” marks on the walls which are used by the crew keep to help keep track of what they have already finished checking.