(a) 35000 N (b) 1.944 m/s^{2} (c) 15552 N

(a) Force exerted by the engine, F = 40000 N

Frictional force offered by the track, F_{f} = 5000 N

Net accelerating force, F_{a} = F - F_{f} = 40000 - 5000 = 35000 N

Hence, the net accelerating force is 35000 N.

(b) Acceleration of the train = a

The engine exerts a force of 40000 N on all the five wagons.

Net accelerating force on the wagons, Fa = 35000 N

Mass of the wagons, m = Mass of a wagon × Number of wagons

Mass of a wagon = 2000 kg

Number of wagons = 5

∴ m = 2000 × 5 = 10000 kg

Total mass (including the mass of engine), M = m + 8000 = 18000 kg

F_{a} = Ma

⇒ a = Fa / ma = 35000 / 18000 = 1.944 m/s^{ 2}

(c) Mass of 4 wagons (excluding wagon 1) m = 2000 × 4 = 8000 kg

Acceleration of the train = 1.944 m/s^{2}

∴ The force of wagon 1 on wagon 2 = = 8000 × 1.944 = 15552 N