On Thu, 21 Jun 2007 13:38:36 GMT, Olof Lagerkvist wrote:

Chesham to Upminster is the longest trip a train could physically take
without reversing or going in circles AFAIK.

As somebody already suggested, Chesham - Aldgate - Tower Hill - Ealing
Common - Uxbridge is much further.

If you count such circular routes we can take for example Upminster -
Earl's Court - over to Piccadilly Line tracks - round the Heathrow loop
- back to District tracks - Earl's Court - Aldgate - Chesham. This can
as far as I can see be done without going over the same track twice or
crossing it's own route (if it is done in this direction, it will cross
tracks already used if it is being done in the opposit direction).

If you're allowed to cover the same route twice (just not the same
actual track), you might as well just do Chesham - Chalfont & Latimer
- Aldgate - Heathrow loop - back same way to Chalfont & Latimer -
Amersham.