Orbirail was a name for a suggested orbital railway route around London.
It would have connected the extended North London Line, East London Line, the Inner South London Line and the West London Line, possibly including the Gospel Oak to Barking Line. The combined line would have orbited London in Zone 2, Zone 3 and (possibly) Zone 4.
It is not to be confused with London's existing circular railway, the Circle line of the London Underground.
Read more about Orbirail: Status, History of Orbital Railways in London, Arguments For and Against