Starting to use the service amounts to entering the contract.

A contract cannot exist unless a fee has been expressly agreed.
This is why Ryan Air charge 1p for its 'free' tickets. Without this
monetary exchange it cannot claim a contract exists
and therefore the terms and conditions apply. It is also why you tick the 'I
have read the terms and conditions' before the payment.

It is also why lots of vouchers have a marked value of 0.001p. A meaningless
value but represents a 'payment' and allows the terms and conditions to
be contractual.

Because the rail system has come out of a government body it may have
special statutes applying that are specific to transport law.
As a private company its terms and conditions are meaningless unless it can
show you have entered into contract with them.

A key point is that both parties understand what they are getting from the
contract:

http://wapedia.mobi/en/Meeting_of_the_minds


Remember Cheri Blair got off from boarding a train without a ticket and
the means to pay for one because
she believed she could pay in Euros, while the rail company's terms and
conditions do not allow for this.
The key point being Cheri Blair is a barrister and the rail company saw
little point in bluffing that there terms and conditions applied.