In Hong Kong, you can get on the subway, pay a few HKD for a ticket from an efficient machine, or buy/recharge a card quickly.
Cross the border and welcome to Shenzhen subway: upon entering the station you see six ticket vending machines. Three are out of service - the other 3 are active, but have approximately 150 people in 3 lines. The active machines only accept crisp 5 kuai bills - not the much more common 10 kuai bill or the common 1 kuai note. There are no windows to get tokens.
There is a subway worker selling subway cards, minimum buy 100 kuai, but there's not much point in that if you don't live in Shenzhen.
The subway worker can give 5 kuai notes for change, sometimes. There's no automatic change machine.
Luckily, someone's noticed this situation - an older woman wearing an apron is helping people at the front of the line. Unfortunately, she has no change - all she can do is talk and push the buttons for them.
The old woman should simply buy a bunch of tokens of different values and keep them in bags around her waist. She should also have a bunch of change, and just sell cards/tokens to people directly. If they would just issue cards in denominations of 20 kuai, many people who go to shenzhen infrequently would still buy them - and buying a card for 20 kuai takes about 2 seconds.
The woman also can't make change for you. (Another instance of the eternal mystery of China: why do people who should be able to make change so often not have enough?) There should just be a big map listing all the stations and the price to get to them. This single woman making 10k USD / year could replace all 6 machines, which are always broken, need refilling with change, and probably cost 50k USD each.
Would just be to allow resellers to buy a bunch of tokens and set up little stalls selling them for slightly more than the official price. This is what they do with phone cards - the 20 kuai phone cards cost 22, to motivate sellers to carry them - and you can get them everywhere! The sellers would make money, and would allow people who were willing to pay for it to save time. I'd rather do that than wait in a pickpockety line.
People do try to make deals with the guy buying tickets, but it doesn't really work because every token costs a different amount. It's hard to buy for someone and improve the efficiency of the machines. i.e. even if I have exact change, and someone in front of me types in their cost & number of tickets, there's no way for me to get them to ask for one more ticket and just give them the money. This is unlike Beijing, where despite the fact that all tokens cost the same there, nobody ever does deals - there's just a line of people who are forced to find a crisp 5/10 kuai note, even though every single one has 2kuai exact change in their pocket, all buying exactly one ticket, and the tickets all cost the same amount, and not doing deals with eachother. It's absolutely insane - everyone has exact change, but they all have to use another denomination to buy it, and nobody realizes this and cooperates.
First of all because China doesn't approve of markets, second because they want everyone to get cards so they can analyze traffic flows on the subway. But not just traffic flows - since tokens already give them info on which stop someone enters/exits the subway station - what they actually want is to be able to profile individual travel patterns. So because of their spying/demographics goals, there is a constant mass of stupidly suffering humanity, 7 days a week, at every single one of the hundreds of Shenzhen subway stops.
Thirdly, tokens don't diplay their monetary value, and are only valid for a day. This makes it so that secondary, more efficient markets can't develop. If they displayed value and were valid for a week, the drink sellers outside the station would simply buy some at the beginning of the week and sell them (with no markup, just for the occasional extra drink they'd sell). This would cost the seller nothing and would massively increase the speed people could get into the station, and would cost the subway nothing either.
What about just buying a bunch of extra minimum cost tokens and then getting on the subway, and then getting off wherever you're going - you could then go through the much shorter, human staffed "fare adjustment" line.