The Kearsley branch was opened on 28th February 1878, constructed to connect with the Bridgewater Collieries. Originally there were two branches: No2 Branch (the northernmost on the map in table38's post) was the shorter of the two and served what my reference book calls Stony Hill Colliery - although it's referenced as Stonehill Colliery on table38's map. This opened at the same time, but only lasted a decade before the closure of the coal workings rendered the line redundant.
No1 branch provided a connection on Linnyshaw Moss (just south east of the M61/A666 junction) with the Bridgewater Collieries Railway by means of extensive exchange sidings. The branch was originally double track from Kearsley Junction, where there were sidings on the Down side opposite Kearsley signal box. Additionally, until the line was singled in 1930, there was another signal box, Kearsley Branch Sidings, which controlled movements on the branch from a 12 lever frame. This box closed around the time the branch was singled, and was replaced by a ground frame which controlled movements to and from the branch. The bottom end of the branch retained two tracks (not sure how far up this continued, but it was single track by the time it passed Roscow Road level crossing on the north-eastern side of Bolton Road at the top of Stoneclough Road), with the southernmost track being used as a headshunt for the sidings at Kearsley Junction.
The sidings at Kearsley Junction were enlarged and a short branch down to the Lancashire Electric Power Company's Kearsley Power Station was laid in 1927. This branch and some of the sidings were electrified.
BR locomotives were only permitted as far as Linnyshaw Moss Sidings Frame at the summit of the line, where the colliery engines took over. Linnyshaw Moss Sidings were closed to traffic on 26th June 1966 and the connection to the colliery removed. The remainder of the branch closed north of Kearsley Branch Sidings on 15th August 1971, but the sidings and a short headshunt remained in use until the power station branch closed in 1977.
So the bridge over the M61/A666 would only have had an 'operational' railway line upon it for eight months - but given that the link to the colliery and exchange sidings were already closed by the time the motorway opened, it seems doubtful if the bridge was ever actually used by anything other than the track recovery train! As to why the bridge is so substantial; I can't answer that. It certainly looks like it is double track, but it hadn't been a double track railway at that point for 40 years. The bridge is two seperate spans - perhaps one carried the railway and the other a footpath or similar track?
