In the steam locomotive era, tram engines had to comply with certain legal requirements, although these varied from country to country: * The engine must be governed to a maximum speed of 16 km per hour (12 in the UK) * No steam or smoke may be emitted * It must be free from noise produced by blast or clatter * The machinery must be concealed from view at all points above 10 centimeters (4 inches) from rail level To avoid smoke, the fuel used was coke, rather than coal. To prevent visible emission of steam, two different systems were used: