Those who read the small print on the official 2FOR1 website interpret that the tickets issued by Stansted Express and Gatwick Express are not valid for 2FOR1. The short answer is that they are valid for 2FOR1, including mobile tickets and they advertise this fact on their respective websites. The Heathrow Express and TFL Rail Heathrow airport train services do not support 2FOR1.

On the discussion below, a common question is, “if I buy a one-way train ticket (often Vauxhall to Waterloo) will it qualify me for 2FOR1?” The answer is not a clear cut one This is a grey area and open to interpretation and judgement and at the end of the day the ticket seller has final say whatever you may think. It's also worth emphasising that different attractions, even different people on the ticket desk at the same attraction perform widely different levels of scrutiny of your tickets. Feedback has been from several people that if refused at one desk, trying at the next desk is fine. If you are contemplating taking this approach because you don't have a need to buy train tickets or Travelcards purchased from the railways, the key point to remember is that the train ticket you buy should be consistent with using the ticket in order to visit the attraction in question. For example if you purchased a train ticket Vauxhall to Waterloo this is consistent with visiting the London Eye next door to Waterloo Station. If you were visiting Kensington Palace for example this train journey is going in the opposite direction of Kensington Palace from Vauxhall so may be rejected by the person on the ticket desk. To skip the hassle of this method, consider using the rail-issued Travelcard if you don't require an official train ticket to London.