Lots of others mentioned key downsides of QR codes that make them generally unsuitable for this purpose (they are stateless, trivially copied, easily fouled from repeated use, too slow because of fundamental optical constraints, require a 100% available centralized ledger/don't work while the reader is offline, etc. etc.) but I'd just point out that decades of R&D have gone into making these systems work well at scale at the busiest train stations in the world, and QR codes came up short. Here is a glimpse of early R&D efforts that went into this: https://www.jreast.co.jp/e/development/tech/pdf_6/tec-06-40-... - it covers some of the technical requirements these systems fulfill.