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A transverse boiler is a boiler used to generate steam to power a vehicle. Unlike other boilers, its external drum is mounted transversely across the vehicle.
The obvious advantage of the transverse boiler is that it is short lengthways, compared to a locomotive boiler. Where space is short, such as with railmotors, this can give more usable space inside. A less obvious, but more significant advantage for road vehicles, is that the water level of the boiler is less sensitive to the road gradient. When descending steep hills, there is less risk of the firebox crown being exposed above the water level and dangerously overheating.
Transverse boilers are broadly similar to locomotive boilers in construction. They have an internal firebox with multiple small fire-tubes.