You are thinking of touch in points only on platforms. Have some imagination and put them on the train and your issue is solved.
1) What’s the penalty for not touching in? A fixed sum? Fare to the final destination of the train? If so, from which station?
2) How do you prevent people only touching in when challenged?
3) What’s the maximum fare for incomplete journeys (touch in or touch out only)? See @najaB’s examples above.
4) How much will the infrastructure cost to install the readers and to ensure that the train correctly registers which station it’s at?
5) What if the train is full and standing and nobody can get to a reader?
6) How do you enforce First Class with this?
7) How do you upgrade your ticket to First Class?
8) How do you monitor abandoned journeys, when nobody has a ticket, just a touch in with no destination stated?
9) Will all contactless touch ins have to be pre-authorised by the bank? If so, what sum do you authorise for? £20? What if their journey’s going to cost £250? Are you going to pre-authorise that? For every passenger just in case? What if you only pre-authorise for £20 and a passenger travels London to Glasgow and doesn’t have the full fare available on touch out?