Hi,
I have applied for some PhD positions and managed to secure 2 offers. Recently i have been invited for another interview, although having 2 options I would like to attend the interview as the position is sponsored by a highly lucrative company and also has a better stipend.

Getting an interview does not mean that I secured the position and the email i received from the Uni asks me to attend the interview without giving details of who will pay for the travelling costs. I checked train tickets and they cost 50 quid. Does anyone have a clue of what i should do?

They should pay. Email someone (preferably an admin person) to check. Likely that you will need to by the ticket then submit an expense claim so it might take a little while to get your money back, but it should happen.

I found some do, some don't. Warwick didn't pay me travel expenses, some cover say up to £50 whereas others cover the lot!