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.
replyForests are irrelevant; rail right-of-way contains no trees, and is wide enough to assure clear "view" of satellites.
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.
reply> Tunnels are controlled by a single block - only one train can ever be allowed access at once, period.
replyAbsolute nonsense. Do you have any idea how long some tunnels are?
There are actually different definitions for what a block is. It can be both a section where you can decide for trains to enter or where you can change their sequence (Zugfolgeabschnitt vs. Zugmeldeabschnitt, don't know the English terms) Also it can be a physical implementation of any constraint between components of different stations.
replyBut yes there does not need to be a limitation on there only being a single train in a tunnel.