You will have much more luck getting in with an agency first who will put you through your PTS and allow you to get some experience.
Agencies can offer construction jobs if you have a CSCS card. These are easier/cheaper to obtain I don't know if the testing centers are still open(they usually share the written driving test sites). I suspect the trick is to find a building site agency, that also works on railway jobs, stick that job out and then they will keep you on and put you through the PTS when they need you on that site. They aren't likely to risk the cost of a PTS and see you jack it in after 3 days if they hardly know you or you've done little similar.
One trick that computerisation should have eliminated from the agency taking back the PTS Card at the end of a job proclaiming it belonged to them and thus stopping you working for a rival Contractor, which happened to me at the turn of the millenium.
Another was a clause in the contract, signed each time a timesheet was filled in with a small paragraph explaining; Should the Rail Company wish to employ the agency worker directly within 'X' amount of weeks then they were to pay compensation to the agency for each staff member.
Thus to happened to me once I'd inquired why all the Agency staff currently working on track failed to gain permanent employment when the vacancies for Railtrack came up. It wasn't our performance as they(Area Managers) would subsequently request the same people for future agency work.