Tunnels are controlled by a single block - only one train can ever be allowed access at once, period. Until the "current" occupant train is verifiably out of the tunnel, that block is forbidden to all.

Forests are irrelevant; rail right-of-way contains no trees, and is wide enough to assure clear "view" of satellites.

> Tunnels are controlled by a single block - only one train can ever be allowed access at once, period.

Absolute nonsense. Do you have any idea how long some tunnels are?

You are just answering how the current system actually works. This discussion was about how a hypothetical system works based on GPS or radar alone.