Regular travellers in London using the Circle Line must often wonder to themselves why the service is so truly awful. Read on …

The Circle Line is cluttered with other services joining and leaving along most of its route – in fact there are only two short stretches where it has the tracks entirely to itself (Aldgate, and the bend between Gloucester Road and High Street Kensington).

The busiest stretch is the south side which also carries District Line trains to & from Ealing Broadway, Richmond and Wimbledon. This means that, with the best will in the world, only one in four trains can serve the Circle Line.

The maximum capacity of the District & Circle tracks is 32 trains an hour in each direction, giving a train interval of 1.875 minutes. If only one in four trains can be a Circle Line one, then the theoretical interval for the Circle would be 7½ minutes.

However, every schoolboy anorak will tell you that:

                         Concurrency =   Existence Time
                                                        Headway

where   Concurrency is the number of trains required to operate the service

             Existence Time is the total journey time, and

             Headway is the time between trains

The Existence Time for the Circle Line is 47 minutes – that’s how long it takes a train to complete one full circle, stopping at every station.

The Headway we already know from above is 7½  minutes.

So,     Concurrency =   Existence Time          =            47                  =          6.26 trains
                                         Headway                            7½

Now you can’t have 6.26 trains, so you have to work the service with 7 trains instead. This means that

            Existence Time = Concurrency x Headway = 7 trains x 7½ minutes = 52½ minutes

And therein lies our problem – to fit in with the District Line services, the Circle Line trains must make the round trip in 52½ minutes, yet it only takes them 47 minutes. So, they must lose 5½ minutes each time they go round. In order not to delay other trains, the master plan is for them to wait 3 minutes west of Aldgate, and another 2½ minutes at Gloucester Road.

Now you know why you get such a lousy service on the Circle Line – only one train every 7½ minutes at the best, and long waits as you go round. And, short of building new tracks, there’s nothing anyone can do about it. In practice you get a far worse service than this because if problems arise (train, signals or track fault) then they miss their slots and end up waiting everywhere.