I don't specifically know about Xi'an, but I remember in 2012 subways in Beijing were a flat 2 rmb (about 0.35 USD) and in Shanghai something like 2 to 4 rmb depending on distance. Nanjing was similar, but I don't remember the specifics. So assuming all Chinese cities that have subways charge the same order of magnitude, this loophole would have indeed been valuable.