From 19th February 2019 Heathrow Express will be accepting Oyster Cards and also Contactless card payments at their barriers.
There is some limitation to this though, in that the tickets won't have any advance purchase nor return ticket discounts. The tickets will also not be subject to TfL travelcard capping, so any spend is entirely supplementary to your normal London commutes.
The costs are:
- £25 for peak times (06.30-09.30 and 16:00-19:00hrs Monday – Friday excluding UK Bank Holidays)
- £22 for off-peak times.
- Free for children under 16 years old when using their personalised oyster cards.
- If you want to travel in First Class you can still tap in and then purchase a supplementary fare on board.
What does remain unclear though is if railcards associated with your Oyster Card will automatically trigger the respective discounts. For instance those who hold 18-25 or 26-30 Railcards are eligible for 1/3 off the Heathrow Express prices when bought at the ticket machines, but whether this will apply at the gates remains uncertain.
The outright cheapest Heathrow Express fares, starting £5.50 each way if you book well in advance, require using Heathrow Express' official smartphone app (then scan the barcode from your phone) or booking via heathrowexpress.com then printing it out.
But there are a myriad of other ways to get to/from London, which Kevin from Economy Class and Beyond covers nicely. Worth reading up if getting to Paddington Station is not convenient for you or you just want to save as much money as possible.