How does someone stop being an Avatar?
"Squashed under a goods train, he was. They were scrapping him off the track like strawberry jam." - Mildred, Tea Lady
Dying is the easiest way to stop being an Avatar, and it is what happens to most Runaways. A preferable way is to stop being the typical example of what you are. In the case of a Runaway, that means to stop running. When that happens, the Avatar leaves the Railway. This is referred to as "finding the right station".
What's an Interface?
"Where we are right now is what the smart people call an Interface. Where two transport systems can join. Like, when the Road leads to a Seaport." - Jeff, the Road Runaway
Transport systems often meet up. For example, you might take a taxi to a train station, and then take the train to an airport. Where the taxi drops you off at the station is an Interface, and when you walk off the train into the airport, that's another Interface.
An Avatar cannot cross an Interface. Their transport system will try to keep hold of them. The sensation has been described as a "freezing electric shock".
Sometimes an Interface is a little blurred, such as the station forecourt where the taxi drops you off. Other time, an Interface will be sharply defined, such as a door from a station which leads directly to the street outside.