Alix wrote in news:973C301C7E1151D7E@:

FOR EXAMPLE:
(1) Get on the tube at South Woodford (zone 4, Central line).
(2) Must travel via Stratford (zone 3).
(3) Get off at Ilford (zone 4, railway station served by One Rail).

Charged for one zone or two zones?

Neither, you'd get a penalty fare because Ilford is a Network Rail
station not served by the underground, and hence only supports Oyster
travelcards, not pre-pay.

You could rephrase your journey as South Woodford (Z4) - Stratford (Z3)
- change to Jubilee - West Ham (Z3) - change to District - Barking
(Z4) so that the whole Z4 - Z4 journey gets done on the tube, but I
think you'd get charged for the incursion into zone 3 nevertheless.

Iain