The way I understand it is that VHF/UHF radio waves posess relatively low energy (compared to light) and therefore are not absorbed by solid objects but simply pass through them. There is indeed some reflection but not of high enough energy to exist for very long; and anyway, the waves are travelling at the speed of light so any 'echo' would be instantaneous.
Unless the tunnel is particularly long/deep so the thicker the wall the more the energy is absorbed and therefore we get radio blackspots in some tunnels.