I think the main problem with the ‘fast train’ method is that you have to obtain a very constant speed, as this determines the matching accuracy of the emission lines. As it is easier to obtain a constant rotating speed, rotating disks have been used, tilted in respect to the direction of movement of the gamma rays, to obtain a constant velocity component in the direction of the scatterer.
Furthermore, some electromagnetic drives have also been made. They will accelerate the nuclei with equal charge and mass at the same rate.
In some cases, also hydraulic velocity drives are used.