The correct answer is option D. "Companies replaced hand-operated brakes with air brakes on trains". The development of air brakes on trains was a significant improvement of railroad safety. Air brakes function much more faster than hand-operated brakes, because hand-operated brakes required that the brakemen turned the brakes in one car and jump to the next to set the brakes on, and so on.
Answer:
Option: D. Companies replaced hand-operated brakes with air brakes on trains.
Explanation:
In the early period, trains manually stopped by involving in hand-operated breaks. The handbrake worked by handling large wheel in the coach, attached to chains and levers which set brake pads under the train. To improve the safety of the rail coach, air breaks replaced because they were much more powerful and reliable, than the manual one.