I am saying how the OP's response looks to GTR. When they say:-
"On this particular day I was not able to get a mobile phone signal at my departure station, but as outlined above, this did not concern me as I knew I would be able to purchase a ticket on the app when I had a signal, so I got on the train as normal. Since there is almost no signal for much of the first half of a train ride to my destination, I decided I would sort the ticket out later in the journey."
is it any surprise that they suspect the OP is not being honest with them. And unfortunately, when they suspect someone of not being honest, then it will throw into question their whole explanation, as to why they were not able to buy a ticket, and may well have triggered the response the OP received, whether or not that was the right response.
"On this particular day I was not able to get a mobile phone signal at my departure station, but as outlined above, this did not concern me as I knew I would be able to purchase a ticket on the app when I had a signal, so I got on the train as normal. Since there is almost no signal for much of the first half of a train ride to my destination, I decided I would sort the ticket out later in the journey."
is it any surprise that they suspect the OP is not being honest with them. And unfortunately, when they suspect someone of not being honest, then it will throw into question their whole explanation, as to why they were not able to buy a ticket, and may well have triggered the response the OP received, whether or not that was the right response.