of journeys in outer London were made by cars or vans, and suggested that improved orbital public transport would help to reduce this figure, proposing a trial of orbital express bus routes linking key suburban rail terminals. Orbirail'''s proponents believe that it would be a relatively low-cost project, involving only a small amount of new track, some improvements to existing lines and an increase in train frequency. It would allow many people to make journeys without passing through Zone 1'', thus relieving congestion on central London's railways. There are complications which could prevent these lines running as single fully orbital route. Orbital railways have an intrinsic timetabling robustness problem. The trains are constantly "in orbit" so there is little scope for "recovery time" if they are delayed. A single delay can have long-lasting knock-on effects and be much more disruptive than on a radial railway. Recovery time can be created by timetabling for longer stops at some stations but this increases journey times and reduces train frequency. (For this reason, on 13 December 2009 the Circle line was changed to a spiral route with distinct endpoints.) In this light, it is hard to see a larger and more complicated orbital railway being approved. An alternative to a single fully orbital route would be two or more semi-orbital routes that join to encircle London. TfL's London Overground is an example of this: in Clapham Junction and in Highbury and Islington, the two halves of the London Overground Orbital Route meet. An additional problem is poor interchange with many of the radial routes. The proposed route offers no interchange with the Great Western Main Line, Chiltern Main Line, the East Coast Main Line, the Great Eastern Main Line or the London, Tilbury and Southend line. Only local services on the West Coast Main Line stop at Willesden Junction. It has similarly poor interchange with many London Underground lines. Poor interchange options are a general problem with most of London's older railways, which were built by competing private companies in the 19th century before the need for a coherent and integrated transport network was understood. References External links London Overground