Wm. Butlers' tar tankers are well-known, a photograph of one frequently being used to illustrate the rectangular tank wagon design.
These tank wagons were used to collect coal tar, a by-product of coal gas production, for refining to extract more useful petro-chemical products. Butlers' operated works in the cities of Bristol and Gloucester and these tar wagons would have travelled widely across the west of England, West Midlands and Welsh borders to collect raw materials from town gas companies to supply the refining plants.
The Butler company originated in the GWR broad gauge era, Mr Butler being appointed by Mr Brunel to run the GWRs' timber treatment plant in Bristol. Later Mr Butler formed his own company, taking over the Bristol works and the company is still operating today as suppliers of industrial oil products like heating oil.