After more than 120 years of continuous progress, world steam locomotive development practically stopped in 1950.
Prior to this time, steam design had seen almost continuous progress ever since the 1830's.
World steam development reached two distinct pinnacles:
U.S. locomotives were supreme in size and absolute power, and in mechanical durability.
French locomotives, attained the highest ratings for thermal efficiency, fuel economy, and power-to-weight ratios.
Since fuel was cheap (coal or heavy oil) and trackage was sturdy to support heavy engines, the "brute force" design philosphy was used.
While thermodynamic improvements were made, emphasis in design was placed on maximum power and durability.
In the 1930's, the first serious challenge to steam motive power appeared in the form of diesel-electric locomotives.
Diesel locomotives and to a lesser extent electric locomotives replaced steam around the world.