Peak and off peak travel
It is often cheaper to travel off-peak (on rail and Tube). However, this is not simple as restrictions can vary between operators. Therefore, always check the off-peak hours on your route before planning your journey.
Generally, on rail services, off-peak fares are available on services arriving into London after 9.30am on weekdays. However, some companies also provide 'super off peak' tickets, which are cheaper and usually available from later in the day. Most train companies operate some evening peak restrictions on some routes. It is confusing and a potential minefield, so please research your particular journey before you book.
On Tube, DLR, National Rail and London Overground services within London, peak hours are between 6.30-9.30am and 4-7pm. If you are using Oyster Pay As You Go (PAYG), you will be 'capped' at the Daily Peak Fare if you touch in between 4.30-9.30am, and the Daily Off-Peak Fare if you touch in after 9.30-4.30am (the following day).
On the Tube, DLR, London Overground and National Rail routes, the off peak fare towards zone 1 is charged between 4-7pm.
If you have a railcard attached to your PAYG Oyster card but only want to use trains in the afternoon peak which are not enough to trigger the price cap it may cost you less to use your railcard and a paper ticket instead.