Compressed air
Compressed-air locomotives were powered by compressed air carried on the locomotive in compressed-air containers. This method of propulsion had the advantage of being safe but the disadvantage of high operating costs due to very limited range before it was necessary to recharge the air tanks. Generally, compressors on the surface were connected by plumbing to recharge stations located throughout the mine. Recharging was generally very fast. Narrow gauge compressed air locomotives were manufactured for mines in Germany as early as 1875, with tanks pressurized to 4 or 5 bar. The Baldwin Locomotive Works delivered their first compressed air locomotive in 1877, and by 1904, they offered a variety of models, most with an 0-4-0 wheel arrangement. Compressed air locomotives were introduced in the Newbottle Collieries in Scotland in 1878, operating at 200 psi (14 bar).