The M&MR was an ambitious proposal to connect Manchester, Northwest England and potentially the English Midlands with the deep water docks at Milford Haven.

[1] The MWR had Parliamentary authorised running rights from Newtown to Brecon via Builth Wells, and hence onwards to Merthyr Tydfil, Cardiff or Neath.

[1] Having moved its junction station for the branchline to Aberystwyth on cost grounds, from Devil's Bridge to Ystrad Meurig (later known as Strata Florida), the M&MR had let the contract for construction of the western 27 miles (43 km) mainline to a combined team of David Davies of Llandinam and Fredrick Beeston but it excluded the route onwards to Llanidloes, as it required additional surveying to overcome engineering and resultant cost difficulties.

[1] The proposal was for the line to head west from Llanidloes by way of: Penpontbren Junction; Llangurig; Pant Mawr; through a tunnel under the present-day Cefn Croes Wind Farm to Blaen Myherin; then to descend down the Myherin valley south-west to Devil's Bridge then southwards crossing the River Ystwyth on a large viaduct at Ysbyty Ystwyth before descending to the revised junction station at Ystrad Meurig.

The same contractors began cutting the route in 1865 and completed the construction of 0.5 miles (0.80 km) of trackbed west of Llangurig station (visible adjacent to the A44 road).

In addition there is a significant civil engineering cutting leading up to the Cambrian Mountains tunnel (south-west of the present-day Sweet-Lamb motorsport complex) under Cefn Myherin.

On reaching this point, the MWR was then compelled by the Act to build another line from here directly to Llanrug, to join up with the M&MHR route, via a 1⁄5 mile (0.32 km) long tunnel under the Cambrian mountains and into Ysbyty Ystwyth.

In the UK it encompassed the collapse of London Bank Overend, Gurney and Company, causing many industrial projects to encounter financial hardship.

In 1882 the M&MR started to dismantle the Llangurig branch, lifting 1.5 miles (2.4 km) of the essentially unused track for maintenance purposes elsewhere.

[3] The unbuilt section between Strata Florida and the railhead of the Llangurig branch would have been through very mountainous terrain, although only 15 miles (24 km) in length as the crow flies.

After emerging from 25 years of bankruptcy in 1900, it was hoped by passengers, freight customers and the authorities that a large railway company would take over the residual western M&MR.

[1] A large part of the route east of Llangurig is marked on current Ordnance Survey maps as "dismantled railway".

[7] The north entrance to Bryn Myherin Tunnel is visible on old maps published in the 1880s marked as an 'old quarry' south east of Cae Gaer Roman Fort at the junction of Nant Ceiliogyn and Pistyll Fawr at about 375m contour.

[8] The south end of the tunnel in the Blaen Myherin glaciated valley appears to be at a similar height 2.2 km south west at a point marked as 'old quarry' on old maps, about 800m north of Blaen Myherin farm, near the junction of Nanr Chwarelmelyn and the unnamed waterfalls to the north.