We are weighing options to get from our hotel near Leicester Sq to Heathrow. Elizabeth Line is new since I've been in London last.
I read that Eliz Line to Heathrow costs GPB 13.30, while the Tube costs about GBP 5.60. But if I tap in at Leicester Sq, and tap out at Heathrow, how does the system know if I took Piccadilly Line, or if I took Northern up to TCR and then Eliz to Heathrow? IOW, what stops me from taking Eliz Line and only paying Piccadilly prices?
Is the answer that at stations where Eliz and Tube both go to, Eliz has a separate ticket barrier that I have to tap in at?
That is correct. I ended up having to take the Elizabeth line from Heathrow twice last year due to works in the Piccadilly line. The trains for the Elizabeth are on a different track area to the Piccadilly trains.
Bear in mind that both fares will count towards the daily cap for zones 1-6. So, if you are planning any other journeys before heading to the airport, the cost of the Elizabeth line route will mean you hit the cap much quicker. Obviously, if the trip to the airport is your only trip of the day, as it often is flying back to the US, then the gap will remain. Equally obviously, if you are doing the reverse journey on arrival it’s more likely that you will do other journeys.
Google up some vlogs on Youtube from train vloggers, they compare and contrast the various methods and means
