At first glance they resemble the codes used by non-UK ticketing systems, but she soon found out that they don’t follow the same standard. The codes themselves are Aztec barcodes, similar to a QR code but with a single central fiducial mark.
These caught the attention of, and she set about reverse engineering their operation. In recent decades these were replaced by credit-card-sized thin card, and now increasingly with scanable 2D codes from an app.
There was a time when to take a British rail journey was to receive a ticket barely changed since Victorian times - a small cardboard rectangle printed with the destination through which the inspector on the train would punch a hole.