If you use Transport for London (TFL) rail services regularly, you could be saving 1/3 of the cost of your off peak rail fares by using a railcard. In the past year, this trick saved me £800. I expect it to save most people who use TFL less but still substantial amounts. Below I explain how.

1. Check if you are eligible for a national railcard. If you are between the ages of 16 and 30, you definitely are. If not, you might still be, e.g. if you are a full time mature student.
2. Buy a railcard for £35/year (sometimes multi-year railcards exist and are cheaper). You can buy directly from national rail or elsewhere, e.g. on trainline.

When you buy individual rail tickets, you can now state that you have a railcard and save 1/3 of the cost. The problem is that you don't buy individual tickets to use TFL services, and so don't get this discount by default. Here is how to enable it.

3. Stop using contactless payment. Instead, get yourself an oyster card either at a ticket machine at an underground station, or online.
4. Make a journey using the oyster card. Remember what this journey was (i.e. the start and end stations, and approximate time).
5. Wait a day.
6. Register your oyster card online. You'll need to cite the journey from step 3 in order to do so.
7. Take your oyster card and your railcard to an underground station, and ask a member of underground staff to link your railcard to your oyster card. Yes, seriously - you really do have to talk to a human in real life in order to do this. Do not ask me why.

Linking your railcard to your oyster card automatically grants you 1/3 off all TFL rail travel during off-peak hours.

Some further notes: - I expect my savings to be higher than most as my routine historically was such that I commuted off-peak in both the morning and evening, so benefited from the discount on every single journey. - The only way to confirm your railcard is linked is to go to manually top up your oyster card at a physical ticket machine, and to check "Discounts: NR Railcard" appears in the top right. You can't check online. - You can enable auto-top up online, so that you never have to think about topping up your oyster card. - No, sadly you cannot add an oyster card to your google wallet or apple pay (yet). You need to use the physical card. - Try and stick to only using the oyster card, and not switching constantly between contactless and oyster. This is because they have separate daily and weekly fare caps. - If all your travel during a day or week is off-peak, your cap with a railcard is significantly lower than without. However, if a single journey is on-peak in some time period, the cap reverts to the regular cap. - When your railcard expires and you replace it you will need to remember to go and re-do step 7. If you forget to do this, I have had some success emailing TFL and asking them to backdate, but they only retain 2 months of travel records, so were not able to entirely refund me.