If you need the source to have a certain velocity you need more space for your setup. Also a problem is that this may increas the average distance between the source and the absorber, which leads to less gamma on the absorber since they are emitted in all directions. It also requires you to make a braking mechanism for the train if you use solids as source. An idea is maybe to use a gas as source and to accelerate it by creating a vacuum in direction of the absorber. Near the absorber put a barrier which prevents the gas from hitting the absorber, but which doesn’t stop the gammas.