A freight train consists of two 8.00 \times 10^5~\text{kg}8.00×10 ​5 ​​ kg engines and 45 cars with average masses of 5.50 \times 10^5~\text{kg}5.50×10 ​5 ​​ kg. What force must each engine exert backward on the track to accelerate the train at a rate of 5.00 \times 10^{-2}~\text{m/s}^25.00×10 ​−2 ​​ m/s ​2 ​​ if the force of friction is 7.50 \times 10^5 ~\text{N}7.50×10 ​5 ​​ N, assuming the engines exert identical forces? This is not a large frictional force for such a massive system. Rolling friction for trains is small, and consequently trains are very energy-efficient transportation systems.