While on holiday in September 2021, I was reading older copies of the magazine BackTrack from the turn of the millennium, from, at that time, Atlantic Publishers. (More recent editions are published by Pendragon Publishing.)
Volume 14 No. 3, March 2000 included an article by Jeffrey Wells  about the Micklehurst Loop (p142ff). Wells highlighted the congestion which led to the development of the LNWR line between Huddersfield and Stalybridge which was opened in 1849. The single-line ‘Nicholson Tunnel’ was the first impediment to the free flow of traffic. This was rectified with the construction by 1870 of the ‘Nelson Tunnel’. “Both tunnels were in use by 24th April 1871 following a period extending from the previous February when only the ‘Nelson Tunnel’ was in use during repairs to the ‘Nicholson Tunnel’.” ( p142)
Wells goes on to explain that ongoing problems with congestion between Stalybridge and Diggle led to alleviating alternatives being considered. Quadrupling of the line was ruled out on grounds of inadequate space.
The LNWR first addressed the length of line to the West of Stalybridge when it opened (in 1876) a line from Heaton Norris to Guide Bridge. It then decided that the construction of an alleviating relief line between Stalybridge and Diggle was the only feasible solution to congestion. The Act authorising the construction of the relief line received authorisation on 3rd July 1879. The route was in two parts – Railway No. 1 was the Hooley Hill Line “which left Denton Junction and joined the MSLR at Dukinfield Junction and Railway No. 2, the Micklehurst Loop Line stretching from Diggle to Stalybridge.” (p143)
Later, the LNWR opened its Stalybridge Junction Railway (1st August 1893) which provided a first link from Heaton Norris to Stalybridge.
The cost of the Micklehurst Loop was estimated at £213,000. The successful tender from Messrs. Taylor and Thomson of Manchester was £177,949 8s 2d. The work was completed and the line opened on 1st May 1886.
Wells talks of three utilities being connected to the Loop. …
The first was a 3ft gauge tramway which served the construction of high level reservoirs. Exchange sidings and the tramway were completed in 1908, “six contractor’s locomotives plied between the sidings at Roaches and a suitable stopping place short of the site.” (p146)
The second was the allocation, in 1916, of 26 acres of land between Stalybridge and Mossley for the construction of a power station. The plant finally opened in January 1927 and Millbrook Sidings were enlarged to accommodate a number of sidings. In addition, “In the 1930s the coal was moved from the sidings by a conveyor which passed under the line. This was later followed by an overhead steel-braced conveyor which stood on tall concrete piers.” (p146) In the summer of 2021 part of the conveyor structure remains standing as does the cavernous goods she’d which graced the sidings. In 2021, the sidings area at Millbrook was heavily overgrown with substantial trees having colonised the site. Plans were afoot for redevelopment of the area and some clearance and regrading had taken place.
The final utility which Wells points out was connected to the Loop was Mossley Corporation Gas Works. “The Works had its own internal rail layout and a complement of small standard gauge locomotives.” (p146) Movements in and out of the site were controlled by Friesland Gas Sidings signal box.
Some excellent monochrome photographs accompany the article, one of which is included here.
The article is also accompanied by two diagrammatic representations of the Loop line and the other lines referred to in the text. One of these maps is included here.
To finish his article, Wells points out that the Loop line suffered a gradual demise with passenger stations closing in the years prior to the end of the Great War. Rumours of final closure attended every significant maintenance problem on the line as the condition of the line’s brick bridges and viaducts began to deteriorate. The last train was to run on Sunday 30th October 1966 with the line gradually being dismantled in the following ten years. The final portion closed when Hartshead Power Station closed. That portion was dismantled in the late 1970s, 1980s and 1990s.
Jeffrey Wells completed his study of the line in the next edition of the magazine with a short series of pictures of Diggle Station which stood at the Western end of the Standedge tunnels. 
- Jeffrey Wells; The Micklehurst Loop Line; in BackTrack Vol. 14 No. 3, Atlantic Transport Publishers, March 2000, p142ff.
- Vernon Heron; The Micklehurst Loop; in Readers’ Forum in BackTrack Vol. 14 No. 6, Atlantic Transport Publishers, June 2000, p370.
- Jeffrey Wells; Through the Lens at Diggle; in BackTrack Vol. 14 No. 4, Atlantic Transport Publishers, April 2000, p235ff.