[gnode]

This isn't about being caught without cash. Whenever I've gone to London I've used my contactless credit card to pay for transport, not because I don't carry cash, but because it's convenient -- you just hold it near the gate and go through; no need to visit a ticket machine.

A problem with this convenience is that you don't get any kind of physical receipt to prove you paid. Perhaps in my case, a ticket inspector could scan my credit card to check it was used to pay, but in the case of a dead phone, it can't be queried.

[milankragujevic]

It helps to have cards that are bought and topped up online, that last for a month and cost 10€ and you can ride anywhere everywhere unlimited for a month. You just need to carry the card and "tap it" on the NFC reader, and have an ID in case of inspection. Or you can use a phone to buy a "temp" card, except it causes so much trouble nobody uses it, including for reasons of bad chips in the readers that refuse to work with some phones, bad app, bad infrastructure (i.e. no 3G signal and the reader in the bus can't validate your ticket so it refuses it, but the phone app already took your money)...

Technology is great when it's thought out. If not... may the deity of choice be on your side in the bureaucratic maze of modern society intertwined with digital/electronic and paper/human-based data carriers.