There's been loads of discussion about this, but the main issues boil down to things like -
If someone taps in but doesn't tap out, what fare should they be charged? I.e. someone taps in at Leeds. They might have gone to Burley Park (Anytime Single £3.40). Alternatively, they could have got a train all the way to Penzance (Anytime Single £237.10).
A couple of recent threads -
Platform edge doors are very pie-in-the-sky in terms of widespread coverage at every single British railway station, but here’s something that is probably more realistic and will bring much greater benefits: How about a long-term rolling programme to get every single station fitted with ticket...
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Today's budget included an announcement that the contactless payment limit was increasing from £45 to £100. As a lot of rail journeys cost under £100 each way, would it be feasible for the industry to start working towards a nationwide touch-in/touch-out pay-as-you-go scheme?
www.railforums.co.uk
Arriving a little late, but I really think that this objection is a red herring.
I think we are all agreed that on a local network, failure to tap in and out (or otherwise to do whatever the rules are) should result in being charged the maximum fare on that network - possibly plus an admin/penalty charge. So if Merseyrail is a local network, then someone who only taps in will pay the fare from Southport to Chester, plus whatever charge has been agreed as policy.
And it's possible to imagine that there will be other local networks. It's not too difficult to imagine a Liverpool network stretching out to Warrington, and a Manchester network starting at Warrington and continuing to Stalybridge.
So do we treat this as one network, but with a default fare from Chester to Stalybridge (via Liverpool and Manchester)? No: we treat it as two networks - but we make sure that everyone travelling from one network to another has their tickets (more accurately, their credit/debit card) checked when they leave one zone and enter the other. We could do that by turfing everyone out of the train at Warrington Central and making them go through barriers before rejoining the train: more realistically we do it by making sure that all inter-network trains have a thorough ticket check at or around the change-over point. So someone who entered the Liverpool network at Bank Hall without tapping in but is travelling to Manchester via Hunts Cross and Warrington will have their card checked at or near Warrington so paying the Liverpool default fare, and will then either tap out at Manchester (so also paying the Warrington to Manchester fare) or hop off without tapping out at Irlam (so also paying the Manchester default fare).
For sure, doing a full ticket check at or near Warrington won't be easy (one inspector on the train? Or a whole team boarding at Warrington and then hopping off at Irlam?). But if the railway isn't prepared to accept the cost of enforcing compliance then they will presumably be prepared to accept the cost of passengers not paying the right fare.
For completeness, the same can be applied even more easily on long distance (so, in practice, limited stop) trains. If you get on the Leeds to Penzance train without tapping in or out, you will indeed pay the anytime single fare of £237.10 (plus penalty/admin fee as per policy). But realistically very few people will be travelling that distance on a whim: most passengers will have pre-planned their journey and will have bought some form of advance ticket which they will be able to show as a piece of cardboard or on their phone.