The Circle Line runs for 99% of its length on tracks built for the District Line and Metropolitan Line, with only the connections at Aldgate and Kensington being dedicated Circle Line track. Because of this, it's often faster to get either Hammersmith and City, District or Metropolitan trains, as they run more frequently.
A circuit of the Circle Line takes about an hour, and passes many of the major London main line terminuses. As Earle notes, directions can be annoying, as London Underground saw fit not to describe the directions in terms of being Circle Line Clockwise but instead as north, south and so on, so that trains change direction a number of times on their journey.
A downside to the Circle Line is that, by nature of its circularity, any problem at one of its stations can cause a knock-on effect that will extend around the whole line. Fairly often signal failures cause large sections of the line to be suspended in one or both directions, and the delays are felt even on the other side of the line. --Earle
In theory, trains run on the Circle Line every 7.5 minutes, with a delay at Aldgate to regulate the service. In reality, the line suffers from shortage of stock and drivers so you may wait for considerably longer. The platforms at Paddington can get extremely overcrowded. --Dave
Excerpted from Sharing The Circle:
- London Underground internal documents call the anticlockwise track "Inner Rail" and the clockwise track "Outer Rail"
- The Circle Line trains circle continuously for most of the day
- The Circle shares track with the District, Metropolitan and H&C lines, so the timetables on these lines are entwined
- The Circle contains six flat junctions where trains on the other three lines join and leave the Circle
- Trains joining or leaving the Inner Rail must cross the path of trains on the Outer Rail
- Trains on the other three lines can terminate at many points on the Circle
- There are also emergency turning points with very limited capacity, used during engineering work closures
Sharing The Circle also has the definitive timetable of Circle Line services.