"Aller bon train" means "going fast", "propagating fast". As "train" also means "speed".

The meaning is not relative to what you’re talking about: so this is not "spreading freely" or "going smoothly", as far as I know.

The opposite is "aller son petit train"… which means "going slowly but quite surely".

A reference here or here.

In complement, we also have "mener grand train" which can have the same meaning as "aller bon train" but which is more commonly associated to the "train de vie" (high quality of life, spending many money).