Horses, steam locomotives, and technological obsolescence

Internal-combustion vehicles did not eliminate military use of horses; US Special Forces could still use them where Afghanistan's terrain made them practical. Steam locomotives offer a more contested example. Their near-disappearance from ordinary railway operation supports the case for technological replacement, while the Darjeeling Himalayan Railway and the Harz mountain railway supply exceptions in revenue service. Diesel and electric locomotives also preserve the railway's transport function even as they replace its source of power. A new propulsion system can replace a mechanism while leaving the wider platform useful.