Dear Harlemswife,
Going by the location, I think it's highly likely that your ancestor worked at the St. Helens Junction sheet shop, known locally as the sheeting sheds. It stood on the south side of the line, west of the station, accessible from Penlake Lane and via a footbridge over the railway from Monastery Lane. The site was later occupied by the Penlake Industrial Estate, but as far as I can tell from Google Maps, nothing now remains.
St. Helens Junction was one of the bigger sheet shops, employing 182 staff in July 1945. But by the early 1960s, demand for wagon sheets was in decline and St. Helens closed on 31 July 1963, activities transferring to the former Lancashire & Yorkshire Railway works at Horwich. St Helens staff were given the opportunity to transfer to Horwich, but few took up the offer. The building was subsequently sold to N Greening and Sons Ltd.
You will find something of the history and activities of the sheet works here
http://www.suttonbeauty.org.uk/suttonhistory/transport/. This tells us that the works was almost completely destroyed by fire in 1874, but obviously it must have been rebuilt to continue for a further 90 years.
So far I have been unable to locate any decent photos of the sheeting sheds. The nearest thing I can find is here
http://www.suttonbeauty.org.uk/suttonhistory/sutton_memories17/. In the entry "My Penlake Memories" the end of the sheds are just visible in the background.
I hope this is of interest and shall let you know if I discover anything more.
Cheers,
Phil