Louise Evans from business advisors Grant Thornton says promotion could be worth up to £150m for the club and will have knock-on benefits for the city and Wales.
More top news
Workers on the First Great Western service are set to walk out from Wednesday evening until the start of services on Saturday morning.
It's a dull and dreary start to the day for much of Wales, but hang on in there - things can only get better!
Some very wet weather will sink it's way south and east overnight, with mist and murk developing around coasts.