Nothing wrong with 'customer' with regard to railways. It can be argued that you are a customer during the ticket purchase transaction and only become a passenger after purchasing said ticket and entering the rail network. Once onboard you remain a passenger but could become a customer again if you visit the buffet or deign to make a purchase from the trolley.....
Passenger or Customer, when all is said and done, is such a minor issue. There are far bigger fish to fry. I used to worry about such things, being a bit of a pedant, but I've come to realise that, in the great scheme of things, how I see myself, or how the railways see me, is of little import. As long as I'm paying my way and the railways are honouring their side of the contract, it matters little whether I'm seen as (or see
myself as) a customer or passenger.
On the flip side, if I ain't paying my way then I'm a 'fare dodging scrote', and if the railways ain't honouring their side of the contract then..... well, there's all sorts of names I could call them.
