Your argument appears to be that a disabled person who is unable to use machines boards a train, they are entitled to buy on board, without penalty, from a member of staff. However, if they successfully purchase a ticket online while on board, they are committing an offence, even when they had no requirement to buy online.
In other words, you accept that a disabled person has the right to board the train without a ticket, but only become guilty of an offence if and when they buy a ticket online.
I'm not entirely convinced by this argument. You may be right, but you are presenting it as absolute fact, rather than your opinion. Do you have any evidence that this is definitely the legal position?
Is your post based on working for a legal firm, or having studied/knowledge of disability legislation, or any other area in which you would acquire such knowledge?