[raincole]

In Taiwan, there is a similar system called EasyCard. That works offline.

...And as you expect it is vulnerable to double spend attacks. Hilariously the vulnerability was revealed in 2014 and nothing has been done to mitigate it. Yes, you can double spend in Taiwan today if you don't mind risking jail time.

From my (rudimentary) understanding of CAP, there is no prefect solution to this. I wonder how Japan handles it.

[account42]

You can also "double spend" cash with fake bills. Zero fraud is never a reasonable design goal.

Much easier than double spending would be to use a student or other reduced fare card.