"Being prosecuted for allegedly attempting to use husband's Season ticket (but did not use it)"
There are two factors to look at here:
1. As has already been posted, Section 5(3) of The Regulation of Railways Act opens by saying 'If an person travels, or attempts to travel....' so the attempt makes this a prosecutable offence
2. This has been posted many times on here previously, but is relevant again in this case. The Appeal Court (1946) was asked to consider the case of Arthur Browning, who presented his wife's season ticket for his journey. The Appeal Court ruling made clear that ' whilst the railway company may not have lost any money, Mr Browning had not paid HIS fare ' and upheld the conviction.
The point is not the law here.
It is the definition of 'attempts to travel'. That elemnet of the charge would have to be proved beyond a reasonable doubt for a conviction.
I struggle to see how an enquiry 'can I use this ticket' can have enough weight to establish the element of 'attempting to travel'.
Many enquires like this are made every day hundreds of times on the railway.
Take for instance passengers stood on Doncaster platform where there are two open access operators and one TOC that serve London.
All day long staff are asked can I board this Virgin service with this ticket. When the ticket is examined it is for Hull Trains or Grand Central and not valid on a Virgin trains service. The person is then told they cannot as it is valid on a Virgin trains service only.
It would be perverse to suggest that those people are 'Attempting to travel'.
The burdon of proof would lie with the TOC to prove attempting to travel.