Not necessarily true. I had the same problem when I wanted to photograph my local station as part of a 'then and now' project I was doing for my personal enjoyment.
I walked into the station and told the ticket clerk, 'I'm just going on the platform to get a couple of photographs for this project...' and explained it to her. She said, 'You can't do that.' When asked why not? She simply said 'Security.' When pressed all she did was repeat, 'You cannot photograph on the platform because its a security risk.'
So I came home and wrote an email to her company's head office which said:
Dear Sir/Madam,
I am making a then and now photographic project of my hometown, using 100 year old photographs and taking them again from the same spot in the present day. I have attached a couple of examples.
On requesting to take a couple of photographs of the platform at Shoeburyness station I was refused on the grounds of 'security'. I realise that in this post 9/11 world we all have to be careful, and that Shoeburyness station is probably high on Osama bin Laden's list of priorities, but is your company seriously saying there is a ban on photography on it's property?
If this is so, are you stopping all trainspotters, tourists, amateur photographers etc. from using cameras on your property?
Yours etc.
Two days later I received a reply basically saying,
'There is no ban on photography on our stations or trains, we only ask that tripods are not used on platforms and that the progress of passengers is not impeded.'
I believe that basic rule goes for the majority of Britain's rail network.