The acceleration, in ms^{-}^{2}, is defined as the rate of change of velocity, per unit of time. To calculate the trains acceleration, we use the equation Acceleration= Change in velocity/ Time Taken. For this situation, the change in velocity is 50-10= 40 m/s. The time taken is 20 seconds, so the answer is simply 40/20 = 2 ms^{-2}.