It's not any use to them, though - you can't reclaim zero VAT.
Agreed. I think it's more because an invoice has a standard set of information on it in a standard format.
That's pretty bizarre it has to be said. It sounds like they don't really understand the rules to me.
I understand it to be because the ticket doesn't actually show the purchase of the ticket and confirmation of payment, nor show the full amount of the transaction (which could involve multiple tickets).
The ticket is the 'thing you have bought' - permission to travel. It's not intended, and does not show, the payment for the 'thing' or the VAT status.
(Just to explain this a bit more - it's like submitting a picture of a book as proof of an expenses claim. The book has a price on it and all the information about the book but it doesn't show who you bought it from, the actual price paid less discounts, how you paid, when you bought it etc.)
It is not the railway's requirement to pander to overly pedantic third parties - such third parties need to respond to reality
I don't think this attitude is particularly helpful, to be honest. If people find it difficult to claim expenses on train tickets then they just won't use the train for business travel.
I'm not saying that is the case (nor that my views on invoicing are correct) but there sometimes does sometimes seem to be a attitude that the railway is always right, and everyone else can dance with its tune. That's not a great attitude to have imo.
I would be happy to concede that a ticket on it's own is enough information for an expenses claim, if the railway feels strongly that it is, on the publication of some kind of document that explains why this is the case and how finance/account departments should handle it. If the railway doesn't always want to produce standard form invoices, I think it should be incumbent on them to explain how what they do provide is sufficient. Usually the response to "we don't want to provide an invoice" is "well we're not paying you then"...