Transport systems join up with each other, and with the world outside transport systems.
The places where they join up are called “Interfaces”.
Most interfaces are obvious – if you pass through the doors of a Railway station,
you’re no longer in the world of the Railway – you’ve passed through an interface.
In the same way, if a train offloads goods at a Sea port for transport by Sea, that’s
an Interface too.
Because an Avatar “belongs” to a particular transport system, they can only pass
through an Interface to another transport system if there’s a reason for it. For
example, a Driver from the Roads can’t normally go on a ship, but if it’s a roll-on,
roll-off ferry intended to carry cars, then they can, because for the duration of
the journey, they’re where they belong.
Transport systems are intelligent, and only let people and things that they want
pass through an interface.
It’s a bit like a body’s defence mechanism. This means that if an Avatar tries to
cross an Interface when a transport system doesn’t want them to, it’s bad news for
the Avatar. They’re unlikely to get across the Interface, and the attempt will be
very painful. If they do manage to cross the Interface, then the process will kill
them – unless the transport system wants them to pass.