If caught, there would be a heavy price to pay so it simply isn't going to be worth trying it on.
Would there? The way I see it is that the OP has been given a railcard that they shouldn't have been given. However, now that they've got it, until such time as the railways choose to take it back (which is their right), the OP would be doing nothing wrong by using it, because the railcard in and of itself is valid.
To me, it's a bit like applying for one of those free three-month family railcards courtesy of a newspaper, but being sent a normal one-year one by mistake. (This is a made-up scenario, not something I've actually heard of happening.) The customer shouldn't have received the item, but once they have, it is in and of itself a valid product.
Of course, this is all my opinion, so I wouldn't recommend the OP uses it just based on my thoughts. I'd be interested to know, though, how the railways could prosecute someone for using a wrongly issued gold card, particularly if it's clear that the customer isn't even aware of the fact that they've been wrongly issued it (i.e. they think it's just an added bonus). It isn't a customer's responsibility to know they shouldn't be given something, after all it's the railway's responsibility to not give it.