The Mawddach Trail follows the nine miles of the old railway line from Dolgellau to Barmouth which was in operation from 1865 until it fell victim to the Beeching cuts exactly a hundred years later in 1965. It is, without doubt, the most popular walk/cycle path in the area, one of the main reasons being that it is FLAT (a rarity in this area). This makes it suitable for all the family, whether on foot on on two wheels (or even a uni-cycle which I saw once). It is also very beautiful with tree-lined sections, river views, mountain views, reed beds, wooden bridges, birds galore, an estuary, sea views and even a pub.
The Mawddach Trail (or "Llwybr Mawddach" in Welsh) was featured on Julia Bradbury's "Railway Walks" series on the BBC a few years ago and is often mentioned as one of the best walks in Wales.
The best starting point is the main Dolgellau town car park (next to the Marian playing fields). You can, of course, walk from Barmouth, but I think walking towards the sea is better than walking away from it. In fact, I've never even done it that way.
The first section of the walk is actually beside the old railway line, as that is now the A494. You start by leaving the car park (at the playing field end) and take the signed path beside the river Wnion until you reach the new pedestrian/cycle bridge (built in 2012). Cross this and turn left and carry on until you reach the main Tywyn road. Cross this (carefully) and you're at the "Bont y Wernddu" car park which is where you start to see signs of the old railway line (which you have now joined). The first sign is the bridge you cross before you reach the gate. If you want a little detour, just before the bridge there is a little path to the right which follows the river for a couple of hundered meters (which is covered in wild garlic in the spring). It's always worth stopping on this bridge and peering down the river, as I have seen Kingfishers here in the past.
From here, it's a straight tree-lined walk to Penmaenpool, with reed beds either side (some of the largest in Wales and a Site of Special Scientific Interest).
Go through the cycle-gates at the end of this section and you're at Penmaenpool… home to an old wooden toll bridge which was built in 1879 and worth a short diversion and walk to the other side. There is also a GWR signal box and the aforementioned Pub (The George III).
Once past Penmaenpool you enter a large cutting, before a dead straight section which ends with the first glimpse of your final destination… Barmouth Bridge. Follow the route and you start to see the estuary opening up with herons and oystercatchers and maybe a glimpse of an Osprey which has been known to make the odd visit.
Carry on and you can make another diversion down to a little pond at Abergwynant, if not, then carry on to your next old 'station' at Arthog. This can also be your departure back home if you want, as there are busses which run back to Dolgellau from this little village, just follow the road to your left and turn right until your reach a bus stop. If you want to carry on (and why wouldn't you) just follow the path straight ahead after you go through the gates and this leads you to Morfa Mawdach station (which is the main Cambrian Coast line and still in operation). Here you turn right and carry on to Barmouth Bridge (which was opened in 1867).
If you want a prettier detour at the end, take the right hand path four-hundred meters after Arthog and follow it through the tall trees and reeds. Turn left after the gate to reach the estuary (only use this one if you are on foot, not on a bike). Follow this path BEHIND the row of houses (Mawddach Crescent) before rejoining the banks of the estuary. I prefer this route as it bypasses the station and gives you a better view of Barmouth, the estuary and a side view of Barmouth Bridge. Cross the style over the fence to rejoin the trail and cross the half a mile long bridge.