distinctive GWR safety valve cover, not a dome). Ultimately, the restrictive British loading gauge was a major factor which determined the size of the dome, with large-boilered express locomotives, such as the LNER Class A1, only having space for a very shallow dome. Traction engines Traction engines in Britain were rarely fitted with a steam dome. As their cylinder block was mounted directly onto the top of the boiler barrel, the casting for this was cast with a large steam jacket around the cylinder(s). This jacket was large and tall enough to act as a steam dome. The base of the cylinder casting was a curved saddle to fit directly onto the boiler. Holes drilled in the boiler allowed steam to pass through, usually more than one smaller hole, to avoid reducing the boiler strength. The passages of the steam jacket were large enough that the safety valve could also be mounted on the cylinder block. The regulator could also conveniently be built into the casting, immediately between the dome passage and the valve chest, without requiring long steam pipes. US traction engines were commonly fitted with domes, as were the British-built Howard engines. Stationary and marine boilers Most designs of stationary boilers did not generally require a steam dome, as they were built large enough to allow adequate steam space within their main drum. Water-tube designs had a suitable steam drum mounted high above their evaporating surface and this performed a similar function. Some designs retained a steam dome: German and French practice often fitted them to Scotch marine boilers and other naval boilers such as the Normand, where British practice would do without. Where Cornish boilers were unusually fitted with a dome, in Cornwall this was known as a 'Dolly Varden', from the stovepipe hats forming part of the local women's traditional costume. The boiling water reactor uses a steam dome at its exhaust. See also Sandbox (locomotive) - on steam locomotives this was also generally a dome atop the boiler References Dome Locomotive parts