If a contract is not entered into how can a ToC force anyone boarding at an unmanned station to pay?
When a traveller purchases a ticket, s/he has entered an 'EXPRESS CONTRACT' with the rail operator. Assuming the traveller has paid the correct amount, then the transport provider is obliged to convey the holder of the ticket from the station of origin to the intended destination station, nowhere else and nothing more.
When a traveller boards at a station without facilities an 'IMPLIED CONTRACT' already exists and from the TOC perspective, that contract confirms that the rail operator will convey the intending traveller on their intended journey from the station at which they boarded to the intended destination station in return for payment of the correct fare due. That fare is due in full at the time of travel and not later.
For the traveller, the implied contract confirms that the traveller will pay the fare due on demand (failure or refusal to do so may result in a criminal charge) and that the traveller has with them the acceptable means to pay that fare at the time of travel. Both parties are bound by the terms of that implied contract.
We can argue 'until the cows come home' about what the TOCs should spend their money on and I agree that far better technologies are available than those currently used by many, but it is still up to the vendor of a service or goods, to determine what payment methods they will accept and again I agree that the TOCs could do better in advertising what is and what is not acceptable, but they will point out that this is advertised in their terms & conditions.
The one method that they cannot realistically reject is UK cash.
They could easily all do a hell of a lot better by simply printing what is and what is not acceptable on timetables and at the same time investing in the technology that allows them to collect more of the fares due.
Just yesterday I put a proposal forward that would probably prevent as much as 90% of what is usually treated as Railcard fraud, but I'm not going to hold my breath