ScotRail has led the way with a genuine "Part-Time" season ticket. You choose a specific number of days per week (e.g., Tuesday, Wednesday, Thursday) for a fixed 6-month period. This is cheaper than a Flexi ticket but offers zero flexibility—you cannot change your office days.
Hence the was born—specifically the "Flexi Season Ticket," alongside improved digital "CarNet" tickets and regional pilot schemes for part-time workers. train season ticket new