> 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?

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.

But yes there does not need to be a limitation on there only being a single train in a tunnel.