If the ‘train’ moves towards the scatterer, at one point it will have to stop or deviate from its path such that the source-nuclei do not collide with the scatterer. You would need a way to eliminate the nuclei that have emitted the photon as they no longer need to be on the train. Also nuclei that did perhaps not de-excite yet should not come to close for change of collision. So you need to keep the nuclei until a distance away from the absorber, but the photons should be allowed to go further.
I wonder if perhaps working with an electric field, or something in that spirit, could protect the scatterer from the charged nuclei?