The increase in momentum of the source is equivalent to a decrease in the momentum of the scatterer. So, the recoil energy of the scatterer is reduced if the source would move at the speed of light. There is some limit on how fast the train can go and the doppler effect could also affect the energy of the source. To make this experiment feasible, we would have to overcome the doppler effect with a source with a broad energy spectrum. By measuring the spectrum of scattered gamma rays, it is possible to determine the contribution of the Doppler effect