The firemen make up the engine crews alongside the drivers. They together keep their respective engine in control and provide company, in addition, a voice of reason when an engine is being difficult.
If the drivers are unwell or are called away for some reason, the firemen usually take over and drive the engine instead.
These crew members are responsible for making sure the engine is properly fuelled and maintained; they stoke the fire by shovelling coal or tossing wood and keep the water level where it needs to be. When the water is running low, the firemen fill up the engine's water tanks, which can either be or not part of a tender. After the day's work is over, they clean out the ash from the firebox. They also couple the engine to their train if a shunter is not available.
A North Western Railway fireman's uniform consists of a black tie, a white shirt with a blue jumpsuit over it and a blue cap. But since the CGI switch-over, the fireman wears a blue jacket and dark blue trousers just like the driver.
- To date, two firemen have been named: Sidney Hever, Edward's fireman and Ted, Henry's fireman.
- As with the Ravenglass and Eskdale Railway, the steam engines of the Arlesdale Railway do not have firemen, as the drivers are capable of handling the duties that they do with the larger gauge engines since their cabs are so small.
- Since the CGI switchover, Thomas' fireman has a moustache.
- In the thirteenth series episode, The Lion of Sodor, the seventeenth series episode, No Snow for Thomas and the twenty-fourth series episode, Thomas and the Royal Engine, Thomas' fireman was erroneously referred to as his driver.