A steam locomotive engineered to expand steam twice to improve thermal efficiency. High-pressure steam from the boiler first drives a smaller set of cylinders, and the exhaust from those is routed into larger, low-pressure cylinders to extract remaining energy before being drafted out the stack. While highly efficient, they were notoriously complex to maintain and operate.
Heard on the job
We restored an old Mallet compound, and timing those low-pressure valves is an absolute nightmare.
Also heard as
- compound locomotive
Related terms
Steam locomotive
A railway locomotive powered by the expansion of steam in cylinders.
Firebox
The combustion chamber of a steam locomotive where fuel is burned to heat water.
Blastpipe
An exhaust nozzle inside a steam locomotive smokebox that directs spent steam up the stack to draw air through the fire.