London
Thanks to Transport For London (TFL, formerly London Regional Transport LRT), transport in London are properly organised under a single system, and are rather easy to use, albeit expensive.
The world is your Oyster in London, literally. Much like Hong Kong’s multi-use transport payment card ‘Octopus’, London’s card ‘Oyster’ is also named after a sea creature, and the phrase ‘The world is your oyster’.
There is simply no other easier way to pay other than by card. You touch your Oyster on that yellow disc whenever you board buses, trams and in and out of tube, train and ferry gates. The machines don’t show how much money have you got left on your card very well though, probably to avoid reminding you too much of how ridiculous their prices are.
In Hong Kong, the cheap transport comes at the price of sky-rocketing housing costs. In London, transport costs are so expensive because they have to cover other non-charging transport facilities, say ped(pedestrian) crossings, roads, tarmac, traffic lights and many other items. The Underground, being the most popular mean of transport, is obviously the most overpriced, seconded by buses and trams.
The London Underground have different charges across zone 1-6 (there are actually nine zones), while prices and caps apply as below:
Single:

The prices on the first column are for paper ticket/cash payment, and the two columns on the right are for Oyster/contactless payment. Now you know you should never ever travel in London without a card. There are currently plans to cancel free travel for children.
Caps:

The AUTOMATIC seven-day cap ONLY applies to contactless. It starts every Monday and ends every Sunday. Oyster users will have to purchase a seven-day travelcard separately.
Various discounts are available to students in London and Railcard holders, they are only available on Oyster.
The cap applies also to other services.
The official peak-times are:
06:30-09:30
16:00-19:00
Only applies to Mon-Fri, off-peak fares for holidays and weekends
However, a small grace period is available, but unadvertised. So the
actual peak fare charging times are:
06:35-09:27
16:05-18:57
Peak-time fare only applies if both taps, in and out, are within peak-times.
Yes TFL charge you for staying in the tube for too long.