In 1842 William Marshall bought the defunct engineering works of William Garland and Son at Back Street Foundry in Gainsborough. In 1849 he renamed it the Britannia Ironworks and began to produce road steam engines. In 1857 his son James Marshall become a partner and the company name was changed to William Marshall and Son. In 1861 another son, Henry Dickenson Marshall, became a partner. William Marshall died in 1861 and his two sons continued the business. It was incorporated as a limited company in 1862.

PS Emmylou is powered by a wood-fired 1906 Marshall Sons & Co Steam Engine with a top speed capacity of 8 knots.