FFS...
Let's try an explain in words of two syllables...
Who bought the train ticket? You or your employer?
If it's you - as I suspect it was - then how did you pay for it? Out of your own pcket, I'm guessing, which means out of your income on which you have already paid the tax.
So since the train ticket is not liable to tax as a business expense, you are entitled to get it back.
Hence your employer gives you
back the cost of the ticket
and the standard income tax and PAYE that goes with it (and he gets back the employer's NIC he has paid as well) so in effect you haven't paid tax on the bit of your salary that bought the ticket
Umbrellas kind of roll up these various transactions to keep the books simple, but that's the principle at work.
And if you don't understand such basics, don't even think about challenging those of us who do.