My commute to work starts from a rural station with a card only ticket machine so I get a promise to pay when I pay with cash. I change at a city station with a manned ticket office but my change over is only 5 minutes and it's the other side of the station. The next train after that is half an hour later and will make me late for work. Will my promise to pay be valid on the second train?
Will you need to pass through a barrier line or ticket inspection to change trains? If so, in practice, you may encounter difficulties using the promise to pay to get through.
And are you allowing the official minimum connection time between trains? For example, the minimum connection time at Doncaster is 7 minutes - anything less isn't regarded as a valid connection. In which case, if the official next connection isn't for half an hour, you could be expected to buy a ticket in that time.
It would be much easier to give precise information if you gave precise details, rather than a vague "a city station".