You will be asked to pay the difference between the price of your discounted Ticket and the full price Standard fare (or the Penalty Fare if travelling in a Penalty Fare area) if:
a) you travel beyond the station for which your Ticket is issued;
b) you travel on a train operated by a company other than those listed in
condition 6; or
c) you travel on a route for which a higher fare applies or at a time when reduced
fares do not apply.
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However, if you did it deliberately, intentionally travelling on an incorrect TOC, for example, after your ticket had been endorsed by a ticket office to make these restrictions clear, you could be prosecuted under the Regulation of Railways Act 1889.
And yes, only valid on West Midlands - not Avanti or XC.
If you used Transport for Wales, without changing trains on to another operator somewhere along the way, you could be issued a Penalty Fare.