Horizontal Return Tubular boiler

      Horizontal Return Tubular (HRT) has a horizontal cylindrical shell , containing several horizontal flue tubes , with the fire located directly below the boiler's shell , usually within a brickwork setting.

Horizontal Return Tubular boilers from the Staatsbad Bad Steben GmbH

Vertical Fire-tube boiler

      A vertical fire-tube boiler (VFT) , colloquially known as the "vertical boiler" , has a vertical cylindrical shell , containing several vertical flue tubes.

Taper boilers

      Certain railway locomotive boilers are tapered from a larger diameter at the firebox and to a smaller diameter at the smokebox end. This reduces weight and improves water circulation. Many later Great Western Railway and London , Midland and Scottish Railway locomotives were designed or modified to take taper boilers.