I think many will continue buying off-peak tickets and paying the excess when asked to do so, especially at stations without ticket barriers.
Agreed.
Can RPI's fine passengers or charge penalty fares if they use an off-peak ticket on a peak time train? Or can they only charge the excess?
Nope, they can only charge the excess, and there is no penalty for doing so on-board as opposed to before travel.
I can see the situation at Man Pic all too clearly. STM staff told "no off peak tickets" but
not correctly taught how to do excess tickets.
Also, for anyone holding a 16-25 railcard, it'll become cheaper to buy a full price peak single in the morning, then a railcard discounted peak single in the afternoon.
I won't continue to use the train for my daily journey if the bus is suddenly cheaper, and I doubt I'd be the only one.
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There is no mention of through tickets to Metrolink Zones being restricted by the new rules. So on the face of it you could buy an off-peak return from Wigan to Metrolink Zone E for £7.20. Either tomorrow's announcement by Northern and/or Wallgate booking office should clarify whether that is still valid for return during the evening peak.
A quick search on London Midland's journey planner reveals that off-peak tickets from Wilmslow to Metrolink City are not valid in the afternoon 'peak'. I this will be indicative of the rest of the Metrolink.