There is a problem for a global city such as London in not taking cash, namely that cash is a handy fallback option,particularly for foreign visitors. I work in a supermarket which in an area with a large number of foreign students who come from countries where signature authorisation is the norm. Needless to say these can't be used on self service tills. I imagine it's the same for self service machines. Cash however would be no problem for them. I've experienced a similar situation myself with card network compatibility in the Netherlands.. It turns out a lot of smaller shops (and a few big ones) only accept Maestro (haven't seen one of those in years) and v-pay (never seen one ever!) and are incompatible with mastercard, visa and amex. To be fair this wasn't a problem with transport where at least the former two of these were accepted but tfl will really have to ensure it accepts about every card going (Union pay, jcb, discover anyone?) to ensure foreign tourists aren't put off. And then what to do about signature cards? It's not like a machine can read a signature in most cases. This is just from my experience, it may have been thought about already.