The problem is that China's tourist industry is 95%+ geared towards the domestic market. If one goes up the Eiffel Tower: 80%+ of the tourists will not be from France. If one goes to the Great Wall: 95% of the tourists will be from the mainland. So although the government makes a lot of propoganda noise about TWOV, foreign language signage (both of which are excellent), non-government initiatives, that really go after the bottom line, just don't care about the drop in the ocean which is the adventurous, independent foreign traveller (who would be ones to use Didi/ vending machines etc). The subway is a new one: I can't imagine that cash isn't acceptable at the subway, but I have never been to Xi'an.

tb