Round-topped boiler

A portable engine with round-topped-boiler and parallel-sided firebox
Round-topped boiler from 'Austerity' saddle tank locomotive 3809, removed for overhaul. The 'waisted' firebox is at the far end of the boiler

A round-topped boiler is a type of boiler used for some designs of steam locomotive and portable engine. It was an early form of locomotive boiler, although continuing to be used for new locomotives through to the end of steam locomotive manufacture in the 1960s.