I having this ERROR in my simulation.
It happens between the suballast (smaller spheres) and ballast. The suballast is generated and compressed, them, it should start to generate but the error shows up. I tried to increase and also decrese the dtSafety, but none of them worked.
the error means there is some particle which is moving way too fast (traversing longe distance within single timestep), meaning it is exploding (numerically unstable).
Three things you can try: