Give or take the bugs noted in the other thread, I think the "simplified Trainline" UI used by several TOC apps is a good base, as seen on an iPad Pro. The Avanti app would be my main example, but others are similar. The way the touchscreen UI works should be the same (as close as possible within patents) as an Android or iOS mobile phone, as people are largely familiar with those - it might even be worth considering using Android as the base OS instead of the more usual Windows so the familiar touch UI paradigms are definitely maintained.
Under no circumstances should the 12 hour clock be used (yes, you, Avanti).
Screen should not be too large, 12-14" portrait would seem a good idea, and minimum resolution should be full HD for ease of reading. Those very large, low resolution screens are hard to use up close, particularly for the partially sighted.
Massive, massive bugbear: it is vitally important that there should be no part of the TVM body that obscures above the screen or card machine when the user is standing about 1-2 feet from the machine as is typical to use one. This makes it impossible for a tall person (say 6' 6") with back and knee issues to use the machine, quite apart from the utterly ridiculous layout of these specific pieces of absolute junk[1] which require an average adult user to squat on the floor to use the card machine - a good many users may be physically incapable of this, so I consider that they would fail disability legislation by failing to make reasonable adjustments for those with chronic back or knee issues. I am aware of the conflicts between this and wheelchair users, but by testing extensively with both sets of users it should be possible to have a design that is workable for both (i.e. so both a standing 6' 6" person and a wheelchair user can use the machine's controls without stretching or bending), or even consider having multiple heights of machine as is common in banks of multiple cash machines. Consideration should also be given to the position of the ticket outlet bit along these lines.
There
must be an "open return"/"day return" type option for those not wishing to faff about selecting a return train at that point. The need for this will go away in time if the railway moves to single-fare pricing, but for now it mostly has not.
If a ticket is sold against an itinerary (whether compulsory or not), a printout of that itinerary should be printed alongside the ticket. A bit of till roll would be fine for this. It could be made optional (as per receipts at self service checkouts) if it is felt these would just end up all over the floor.
Nice to haves:
Consider an "expert mode" which a user can select which will issue any walk up fare between the two stations entered for the date selected, but with clear markings (both on screen and on the issued ticket(s) ) that the passenger has made their own choices and takes responsibility for them, a bit like the "restrictions advised" stamp ticket offices use when a passenger asks for something they believe may not be valid.
Consider doing Rovers/Rangers, two search options, one where you type in a station and it shows all of them that would be valid there, and one where you type the name of the ticket. The journey planner should (as the NRE one does) suggest a Rover/Ranger where it is the cheapest way to do the journeys planned.
Consider allowing a passenger to scan an e-ticket and change it/reserve the return journey if not reserved at the time of purchase, including a partial journey (e.g. if you scan an e-ticket from Manchester to London at Birmingham New St, it should allow you to reserve trains from there to London, not only at the start of the whole thing).
Consider allowing seat selection.
[1]
https://www.bristolpost.co.uk/news/bristol-news/bristol-temple-meads-commuters-lament-1604338 - also found at Paddington, sadly.