Personally, I'd mandate:
Staff who previously received commission get the scan bonus. Over time, this naturally ages out as staff are promoted, retire, resign, sacked etc.
New staff or staff who have never received commission - it's part of the job description you signed up, and agreed, to when taking the role.
For both groups, I would give them an additional incentive payment - £250 or so - if they hit a particularly high number of scans over the 13 rail periods. 13,000 scans should be very achievable (1000 per rail period).
If the staff (other than revenue protection roles) proactively flag a particular customer or ticket that results in a fully paid up settlement or successful prosecution, I would give them £50 for each occasion. No conflict or issues with customers needed - as the flagging is a wholly electronic process for most scanning equipment.
There should also be performance management for staff who are way below the scanning average for that diagram/turn/shift - on a persistent basis. TOCs already know the average scanning records and sales per turn number (shift) for each day of the week, for each month. Performance should be reviewed over the course of a year.