Oyster Card vs US Tap Enabled Credit Card for Tube and Trainsin London

They do work in place of an Oyster Card. Not sure about the two fares but someone will chime in who knows. I personally stick with my Oyster Card. If I somehow lose it, it won't be the same crisis for me as if I lost my credit card.

Contactless cards work in exactly the same way as Oyster cards and you save the £7 fee for the latter.
BUT you need a card for each person - you can’t pay for two with one card.
BUT you can also use Phone or smart watch pay as well, even linked to the same account

We set up separate accounts on the TFL website and each registered a “spare” cc we took with us. We kept these two cc in an inside, zippered pocket to use for all of our transportation needs for the 9 days we were in London in 2022. It worked great for us and I would do it again.

https://tfl.gov.uk/fares/how-to-pay-and-where-to-buy-tickets-and-oyster/pay-as-you-go/contactless-and-mobile-pay-as-you-go?intcmp=55539#on-this-page-7

Can you use the same card for overground rail from Easton to Watford Junction to visit WB Stutios? Do you need to book train ahead of time?

Can you use the same card for overground rail from Easton to Watford Junction to visit WB Stutios?

Yes

Do you need to book train ahead of time?

No

Note that depending on where you are staying, the Overgrund could be cheaper via West Brompton or Kensington Olympia.

Staying at London Marriott County Hall. We were thinking of taking the Tube to Euston for the train.

You can do Northern line to Euston, then overground, or Bakerloo to Queen's Park. Wembley Central or Harrow and Wealdstone, changing onto the Overground at any of those. If changing at Euston it is faster to catch a London North Western Train to Tring, Cheddington, Milton Keynes, Northampton, Crewe or Birmingham - Watford Junction will usually be first stop (2nd stop on Tring trains, 3rd stop on Cheddington trains), rather than going all stations on the Overground- same fare.
The only benefit of the Overground is it terminates at Watford Junction so you can't be over carried!

I Might be overthinking this as we are headed to London for 8 days in April. Can I use my Visa CC and tap it 4 times for our family? We did that in NY and it was easy. Tap it once and my kid went thru the turnstile, and repeat for the rest of the family.

No, London is different. In NY there's a standard subway fare; you don't have to tap out, do you? London Underground fares vary with distance, so you need to tap in and out for the fare calculation, and you have to do that with the same card, in sequence (in, out, in, out). A series of taps in wouldn't work. I wonder whether you'd all get charged the maximum fare, which is what happens if you don't tap out. You wouldn't like that number.

Paying by contactless can only be done for adult fares.

Four children aged 5 to 11 can travel free on journeys to and from Watford Junction If travelling with an adult using either Pay as you go using contactless (card or device) or an Oyster card.

Children aged 11 to 15 need to use an Oyster card- [from TfL]-
If you don't live in London and you're visiting with children who don't have an 11-15 Zip Oyster photocard, they can get discounted travel for up to 14 days with the Young Visitor discount:

50% off adult-rate pay as you go fares and daily caps on bus, Tube, tram, DLR, London Overground, Elizabeth line and most National Rail services
50% off adult rate tickets on the IFS Cloud Cable Car and Thames Clippers River Bus services
The discount can be added to a standard Oyster card or Visitor Oyster card by a member of staff at:

Any Tube or London Overground station
Visitor Centres (except Gatwick Airport)
The children must be with you when you ask for the Young Visitor discount.

Note that some US Visa cards and Mastercards will not work for contactless payment on the Tube and other London transport means, even though they are “tap enabled” and work fine elsewhere. This is acknowledged on the TFL website on the page discussing contactless payment.

https://tfl.gov.uk/fares/how-to-pay-and-where-to-buy-tickets-and-oyster/pay-as-you-go/contactless-and-mobile-pay-as-you-go#on-this-page-1

Last September, my husband used his Visa card for “tap and pay” all over Italy, and in London shops and restaurants. But it would not work for the London buses or tube. My AmEx card worked fine, but of course he could not use that if I did. Fortunately we also have Oyster cards leftover from previous visits to London, so he just used one of those. And after confirming that my AmEx card would work, I switched to use the other Oyster card so I didn’t have to pull out my credit card while boarding the bus or entering the Tube station.

That’s good to know for the family. So it sounds like my only option is an oyster card. Or my wife and I pay separately on our Visas and get my kids who are 12 and 15 oyster cards. Does that sound right?

That seems right but weigh up whether the half fares on the Child's Oyster card are enough to offset the fees in getting them. They probably will after a few days. For the adults if t

I get an error message when trying to create an account on the TfL website. Does anyone know if account creation is geo-restricted outside the UK, and I have to wait till I get to the UK to create an account? (I was able to create the account via the TfL app, but get the same error when trying to complete the email verification process.)

Similarly, the Avanti West Coast rail app is unavailable in the US Apple App Store. Is this also geo-restricted outside the UK?

I got the same error on Tfl.

Why do you need an account?

With a TfL account you can add credit to your Oyster at home, rather than doing it at a station, access your journey history and get the weekly Thursday update emails.
Also for dealing with any charging queries.
I managed for years without an account until I needed to submit Company expenses, but now find it invaluable for not having to waste time topping up at a station.
I am a dinosaur in that I won't do tap and go, for previously stated reasons by others, but also feel better protected financially using oyster.

isn31c - I can see why a tfl account would be useful for you but I'm not sure it's worth making effort to get one for someone just coming to visit London for a few days, particularly if it's not straightforward. I have never bothered and I'm down in London a couple of times a month these days.
You can get the weekend updates by email without having an account - I seem to do.

All I know is I started getting TfL e-mails after I set up an account.
To me one of the sells of an Oyster (apart from reduced fares) was that big brother wasn't watching and recording my every journey. I lost that anonymity because of some nonsense to get a single bus fare back from Euston to the Imperial War Museum. Did they think I had walked across London?
If the e-mails can be got by other means so be it.
The other thing is that if you lose your Oyster and have no account that's your money gone. With an account the unused money can be transferred to a new card.
I'm just trying to be helpful and point out the benefits.
If you can't set a TfL account up or don't want one, don't fret.
It's the same with the Avanti app. If it doesn't work, move on.
I never set up an Avanti account when it changed from Virgin, as the new process was so tedious.
Firstly you don't need any app for any train company nor do you even need an account with every train company you are travelling with. Just like the South Western trains purchasing process which seems not to work over in the US for unknown reasons (found the other night) move on and, in that case, book with LNER.
Have one account, and if you insist one app with one train company- LNER seems to be best, but any company will do. Do everything through that account.
I do everything through Northern and, very rarely, Scotrail. That is only to keep my Scotrail Club 50 account alive.
You don't need bar coded tickets on your device. That is a matter of personal choice.
Almost any ticket type that a tourist uses can be purchased as a guest from any train company (with no account), with a collection code e-mailed to you, and then collected as a physical ticket from any machine in the Country. So I could buy a ticket for a Scotrail company from Northern and pick it up from the machine at Penzance.

I will just say we have never set up an account with TFL. We bought Oyster cards some years ago, and just top them up at machines when needed. And as I noted above, I can use my AmEx card for contactless payment if I want.