Welcome to the Forum Deltic009. Finding out about relatives who worked on the railways is quite difficult as so many were employed!
This link suggests that you contact National Archive at Kew which now holds GWR Archives and Records including Staff Records:
http://www.steam-museum.org.uk/steam...oduction-2.htm
It would help if you knew what dates your relative was at Swindon and what job he/she did, otherwise it could be a bit like hunting for a needle in a haystack!
Good Luck.
Best wishes,
John H-T.