Difficulties to achieve the literal description would be in the required speed of the train (around 300-500 m/s depending on the particle’s weight and E0 energy). This is barely accessible speed with any train even today, it might even require sub-sonic plane. The other obstacle is preventing the train from crashing into the scattering medium – the train needs to stop immediately as the gamma ray is emitted from the source.
This might be feasible with long straight train line that is inside a sealed and evacuated environment (with vacuum pumps). This would help to achieve the required speeds easier with much less friction, but breaking might then be even bigger problem. Maybe the trains based on magnetic levitation could be used – no friction = high speeds, very fast stopping speeds.
Another way of doing this might be putting both the source and the scattering media on trains going towards each other, thus reducing required speed for the train with source.