Complex sentence: All three survived World War II and were rebuilt as conventional, unstreamlined locomotives in 1950 with new boilers, in which form they worked until 1957 when electric locomotives took over the high-speed routes.
Split into simpler sentences: All three survived World War II and were rebuilt as conventional non-streamlined locomotives in 1950, with new boilers. They worked in this form until 1957, when electric locomotives took over on the high-speed routes.