The Oyster and contactless PAYG system is designed for single journeys, although a journey by rail can include any combination of National Rail (inc Overground and Elizabeth Line), Underground and DLR. Providing you always touch in and touch out at the ends of your journey you should be charged the correct fare. Each journey must be completed within a set maximum time. If the time is exceeded then the system will charge two incomplete journeys, neither of which will count towards any cap for the day. The amount of time you are allowed varies depending on the time of day and the length of the journey.
As far as Oyster is concerned, the time allowed is related to the number of fare zones crossed during the journey. When it comes to contactless and journeys further out from Central London there are no fare zones, so the time allowed takes account of distance, speed of journeys, frequency of services and the number of potential changes of train required. Sometimes the allowance differs depending on the direction of the journey, usually reflecting a less frequent service on one of the legs.
What is allowed for a specific journey
Given the complexities explained above there really is no set formula for what is allowed for each journey. Thankfully TfL have provided us with the data detailing the allowance for every possible journey. This is displayed on our fare finder underneath the fares for each journey. There are three times, one for weekday daytimes, another for weekday evenings and Saturdays, and a third for Sundays and public holidays. Where there are different routes available for a journey, they all share the same allowance. The allowances for longer journeys may look very generous, and indeed they are supposed to be, because they account for things like just missing a train after touching in, mild disruption and waiting for services in the middle of multi leg journeys. If severe disruption causes the time allowance to be exceeded then the helpdesk will resolve any resulting incomplete journey charges and ensure that the correct fare is charged.
Is there anything else I need to know
The main thing to be aware of is that break of journey is not permitted. If you stop off at an intermediate station and leave the railway then you must touch out. This will end your journey, and when you touch back in again it will start a new journey. Do not be tempted to not touch out if the gates are open or the station has validators. You may be caught out if the gates are closed when you return, or there are staff checking tickets who will ask you to touch on the validator. If you wish to break your journey in the middle then a paper or e-ticket may be a better option. Some train company smartcards also have PAYG functionality on their own services, and these can usually cope with breaking journeys. The Oyster and contactless PAYG system cannot cope with break of journey because the permutations in a station rich area like Greater London are too great.
The exception to the break of journey rule is where out-of-station interchanges are programmed into the system. These allow you to walk between nearby stations, and providing you touch out and back in within the allotted time you will only be charged for one journey. The full list of these interchanges can be found on the osi list page. The time taken to walk between the stations is included in the overall allowed journey time, and if you exceed the interchange allowance it will split your journey in two, even if you still complete the overall journey in time. For example, a journey from Upminster to Aylesbury allows 3 hours and 40 minutes. You might use c2c from Upminster to Fenchurch Street, walk to Aldgate, take the Metropolitan line to Harrow-on-the-Hill and then Chiltern to Aylesbury. The walk between Fenchurch Street and Aldgate must be completed within 20 minutes. If you decided to visit the Tower of London while there you will be charged two journeys, Upminster to Fenchurch Street and Aldgate to Aylesbury, even if you complete the whole journey in less than 3 hours and 40 minutes.
Technical bits
The basic time allowed chunk is 10 minutes and this is multiplied by the number of chunks, then 60 is added to give the time allowed on Monday to Friday daytimes (04.30-19.00). The shortest time allowed therefore is 70 minutes which is usually for a single zone journey in zones 1-9 (eg Crayford to Bexley). This base figure is then multiplied by 1.1 (and rounded up to a round 5 minutes if necessary) to give the time allowed on weekday evenings (19.00-04.30) and all day Saturday. And the base figure is multiplied by 1.2 with rounding if necessary to give the time allowed on a Sunday or public holiday.
Within zones 1-9 the number of chunks is usually equal to the number of zones covered in the journey. If you enter a zone twice then it counts twice (eg a zone 5 to zone 2 journey via zone 1 would be 5-4-3-2-1-2 which is 6 chunks). If there are different routes between two stations then the time allowed is based on the highest number of zones (eg the same zone 5 to zone 2 journey with an avoiding zone 1 option would still be 6 chunks rather than the 5-4-3-2 which might be expected). If one or both ends of the journey is a dual zoned station then an extra chunk is added for each dual zoned station (so zone 5/6 to zone 3 would be 6-5-4-3 even though you don’t go into zone 6). Journeys within zones 1-4 often have an extra chunk or two added in recognition of some longer possible journeys. And some multi leg journeys have also been extended where it is clear that service frequencies mean the normal limit is likely to be breached. As mentioned above, beyond zone 9 the calculations are different.