Charge client VAT on train ticket?

[XLMonkey]

Yes

[VectraMan]

btw, not arguing about whether the client would accept this - in my view its perfectly legitimate to add a markup to expenses that you incur on their behalf. But the point is that it is not VAT, its just profit.

Exactly - it's nothing to do with VAT. There's no reason at all why you couldn't charge the client double the value of expenses, or half, it's just an agreement between the two of you. So it's perfectly legal to do whatever you want, but if you're VAT registered your invoice to them must have VAT added on top.