ScotRail have started to print tickets with QR codes on them, which unfortunately are an awkward size (far bigger than a credit card, even larger than a £20), come as a single strip without a perforated edge to tear, and are generally very low quality. I full expect these to become a common sight on the ground as they're not eaten by machines and difficult to store even in your wallet.
Northern started issuing them instead of cardboard tickets a few years ago and other TOCs followed along after Network Rail/Cubic finished adding barcode readers to ~all ticket barriers.
I understand why: paper barcode-only tickets are significantly cheaper than cardboard tickets (their lack of a magstripe allows any generic thermal printer to be used), but their larger form factor makes them a definite UX downgrade.
