From the quarries at Tyrau Mawr, the railway runs down under Craig Las, along the valley East North East to Ty'n y Ceunant (TYC) farm, a short distance up the Pony Path from Ty Nant car park. This part of the route is fairly straight with a steady drop. At the farm, it is joined by the branch from Llynau Cregennen which runs down an adjacent valley to the North. A passing loop serves the Llynau branch. This is very popular with tourists and is also used by shooting parties for the Bryn Brith lodge. Legend has it that the island in the larger lake was thrown in as a pebble when the giant Idris lost his temper one day.
The direct route from TYC down the valley past Gwernan lake and Rhyd Wen is too steep for realistic working, exceeding the ruling gradient of 1 in 70. Therefore the railway heads North toward Islaw Dref and down the Gwynant Valley, following a large loop under Hafod Owen and winding down through the woods past Kings and the YHA.
Clinging to the contours, the route then veers East again towards the station halt at Penmaenuchaf, an steep climb for passengers from the George III pub and main-line station at Penmaenpool. Half a mile beyond is the junction for Bont Wnion exchange siding where the basalt is transhipped onto the main line spur from Morfa Mawddach, the Ruabon main line having closed in early1963.
The railway headquarters and works are at the Marian Station yard by the River Wnion in Dolgellau. From there, the route runs East under the town bridge and then on for a quarter of a mile to the Cambrian Brewery sidings. Although the brewery traffic ceased when plant closed before World War 2, the buildings were then used by two generations of the Jenkins family who operated a moss wholesale and nursery business from there. The siding also served for animal traffic destined for the town's slaughter house and is occasionally used for funerals at the local town cemetery.
Passing Felin Ship, the line is sandwiched between the river and the Arran Road but then follows a low embankment through the river meadows to the hamlet of Dolgun where there is a passing loop. This is also a popular stop for those tourists who wish to hike up the picturesque Torrent Walk or visit the hotel at Dolserau. There is also an incline and siding serving theTy'r Stint slate working.
This speculative route of the Grogennen Railway was determined after studying the Ordnance Survey maps of the area and excercising a little local knowledge, taking full account of the stock and traffic that would have been expected on inception.