The Circle line is a handy part of the underground network. However, it suffers terribly from reliablity problems. This is in part down to the fact that it has no terminii available in which to have recovery time aside from Aldgate. The other main factor is that fact that it has to intermix with both Hammersmith and City and Metropolitan services in the North, as well as District services in the south (and the District's Wimbledon branch in the west.) This means delays quickly propagate between the aformentioned lines as they are all interdependant on one another. One solution might be to remove the overlapping services where possible, (e.g. District services to Edgware road from Gloucester Road and Metropolitan services to Aldgate), however this would still be insufficent as both the District and Hammersmith & City operate through services which would still need to interwork with the Circle services. Clearly, the only solution for maximum reliability is segregation.

In the late 19th century, the Metropolitan Railway found that due to the increasing levels of traffic coming from through trains from it's mainline links at Paddington, St. Pancras, and Kings Cross that the only solution was to build a second set of tunnels adjacent to their existing tunnels for these mainline trains to use, thus freeig up capacity on their own lines; Accordingly, tunnels were built from just beyond St Pancras's forecourt (with an eye to extending to Euston) along to Moorgate, where at that time the line terminated.

At the time of writing, Edgware Road to South Kensington is operated as part of the District line, but this route was originally built by the Metropolitan, as the District Railway only achieved as far north as High St. Kensington, which explains the unusual paltform relics and arrangements that can be found today. This essentially means that from High St. Kensington to South Kensington there is ample provision for quad tracks. Elsewhere along the District's side of the Circle, there are a few stations which, having acted as terminal stations during the line's development, have had more than the usual pair of platforms: Mansion House and Tower Hill.

Thus, what would be needed to have fully segregated lines would be

A second tunnel from Edgware Road to St. Pancras
A second tunnel from Moorgate to Aldgate Junction
A second tunnel from High Street Kensington to Edgware Road
A second tunnel from South Kensington to Minories Junction
The Thameslink tunnel from St Pancras Midland Road to the Snow Hill tunnel via Farringdon would need to be replaced with a new alignment underneath the existing one. This should not be a problem as the gradients need not be severe and the new route can run under the existing one if need be. This also enables another pair of key objectives, the removal of the flyover east of Farringdon and the reuse of the line from Moorgate to Farringdon.

Concieveably, if the District were to withdraw from High Street Kensington to Edgware Road, then that section of tunnel would not be required. This becomes a viable option when the rest of the route is fully segregated. The scope of this document is constrained to the Circle line's segregation, not the configuration of services formerly using it that are relocated onto the new tunnels. That said, other than running lines obviously consideration must be given to the stations that will offer platforms on the route with the maximum number of stations between stops in the event of two stations having equavilent claim to having platforms.

Whilst the segregation enables the reliability to go up and more trains to be run irrespective of the services running on the segregated services, it does nothign to help the problem of lacking terminal facilities for the services to recover in. To this end, I propose the service pattern set out in one of my RingRail proposals, where the route is divided into quarters, and four overlapping services operate on the Northern (Notting Hill Gate to Monument), Eastern (Kings Cross St. Pancras to Westminster), Southern (Monument to Notting Hill Gate), and Western (Westminster to Kings Cross St. Pancras) sides of the circle. At these four stations additional platforms would be provided to enable every other train to terminate there. Admittedly, the statiosn could have been chosen to make better use of existing faciliies, given the wide-reaching scope of the changes, it seems better to choose statiosn with the best interchanges as the terminal stations rather than the ones that happen to be more convient to build upon. Suggestions of better terminal stations would be greatly appreciated.