Let's assume the origin was somewhere like Bournemouth (that fits with the fare and the journey time).
If he was charged the full fare in accordance with the rules, that's £158.00 but he'd still need to get back. He could perhaps buy a railcard while away from home, but it depends on whether he had the right documentation. That'd be £28. Yes, he could get a refund on the original ticket (£81.50 using Bournemouth as the example) less a £10 admin fee, and then got a £15 Grand Central ticket on the 19:04, and then got whatever the cheapest way of getting SWT back is (or get a coach), but you could be talking about incurring costs of £200! That is harsh. Too harsh, I say.
But he was only charged the difference between the discounted fare that he paid (say £81.50) and the correct fare without a railcard (£123.50) = £42, again using Bournemouth as the example. This is quite fortunate as it also resolves the problem for the return journey. He was also lucky because SWT could have issued a Penalty Fare which would have been twice the price to the next stop (e.g. if caught between Basingstoke and Woking, 2 x £33.30 and would then have to buy a brand new ticket from Woking to York (but, again, would have the problem of having to get back to Bournemouth).
I think you can see now that the guard only charged the difference up to the correct fare that the OP would have paid had he gone to the ticket office and they had pointed out he didn't have his railcard, and was extremely fortunate not to be charged the nightmare scenario of the full fare in accordance with the (very harsh) rules!
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Let me be very clear so you are not confused any further - the customer can ask for any money they paid on the train to the guard over and above the price of the original ticket that was purchased to be refunded to them so they end up being charged as if they had had their Railcard with them on the train that fateful day.
But this is clearly incorrect as they did not have the Railcard!
How interesting that you assume my example of how a court of law may treat mitigating circumstances as directly related to a penalty under a railway bye-law. It was an example. I never mentioned anything about a penalty in my post!
You talked about "passing sentence on a defendant"; clearly that analogy is about someone accused of wrongdoing. This person has not been accused of any wrongdoing, or charged any penalty. That is not the case here. There is no "defendant".
Tesco openly advertise the fact that if you've forgotten your clubcard on one visit, you can write to them with the receipt and get the points back. Never actually tried doing it, but the only difference between that and the TOC's is that the TOC's customers have actually paid for their loyalty cards.
I once forgot to apply a 15% discount to a meal, if I went back later do you think they'd give me my 15% back as if I held the voucher at that time? That is the equivalent. The clubcard points are quite a different issue.
I'm pro-customer; I think the rules are far too harsh. But the OP was treated appropriately in my view and I can see no grounds for complaint; only praise!