London Transportation offers the Oyster Card for buses and subways.
In my opinion - London has the best transportation system in the world. When traveling with 4 or more - London's cute little black cabs make sense. The drivers are well trained - speak excellent English - and do not expect tips.
But for normal everyday one or two people traveling - the buses and tubes are top drawer. They are very clean and efficient. You can get anywhere in the city or suburbs this way.
I like the payment system very much. They have a credit card called the Oyster Card. You buy one and put some cash on it - at the ticket window - at the ticket machines - or on your computer at home. Then - when you go through the turnstile - the little yellow reader both reads your card - but also tells you how much credit you have left. When you exit the subway again - the sensor reads your card.
When you get on a bus - the yellow card reader checks your card and charges you accordingly. It warns you when your balance is getting low - and if you do not have enough money on your card - a red warning light flashes and a warning buzzer goes off.
At home- you can go on your computer - see every spot you used the card. You can add money using a credit card number. It is a very neat way to keep track of where you traveled - also have an account of your travel expenses. If you card has been lost or stolen - you can immediately void the card before someone uses your cash to travel. Remember - there is a documented record of where you were - and when you were there. The might have to end up explaining how someone took you card from your wallet - used it - and then returned it to your wallet without you knowing it.
This morning - I got a pleasing email. Transport of London (the people in charge of Oyster Cards) sent me a letter. It said they made a mistake and they are crediting my card £6.90. That is about $11.00.
If you travel a lot - you can buy unlimited cards for days - weeks - or months. I checked and an unlimited card for a week is £27.00 - which is about $40. Parents can buy their kids cards - and know the money is going for transportation - not other things.