So the MTR is paying HK$2.25 billion for being late. Say it slowly. Two billion, two hundred and fifty million.

Most of us know the feeling of the MTR being late. Five minutes, a announcement, a sigh. Here the lateness was in building the thing, and the bill has now arrived.

Here is how it works. When a big rail line is built, there is an agreed date. Miss the date and somebody owes somebody. Think of the guy who promises your kitchen in six weeks and turns up in six months. Except this kitchen is a railway, and the guy is a company that already runs most of the trains you ride.

There is a second part, and it is the interesting one. The MTR and the government also reached what SCMP calls a consensus on avoiding a payout over delays to the Sha Tin to Central Link. So one set of delays gets paid for, and on the other the two sides agreed to skip the payout.

Now, I run a noodle stall at weekends. If a supplier is late and we both agree he doesn't owe me anything, I still know what happened, because I have noodles on the shelf and he has my money. Here, nobody has explained to the rest of us how the maths works. That is the part worth asking about.

The Sha Tin to Central Link has been a long story. Everyone in this city remembers the delays, and plenty of us were stuck waiting for it to open. Nobody remembers anyone saying sorry at the platform.

HK$2.25 billion is a big number. It is also the kind of number that lands in an account somewhere and gets talked about in a meeting room. The commuter who waited years for the line gets to read about it on the train, when it is running on time.