Don’t miss this hidden gem walk tucked away in the Snowdonia mountains

Start: Lay-by on the A4212, near Llyn Tryweryn (what3words: stack.crouching.scramble)

Distance: 1 mile (to the viaduct and back) or 9 miles out and back to the layby near Trawsfynydd (extended route)

Elevation: 850 feet

Duration: 1-4 hours (depending on your chosen route)

Difficulty: Easy

Navigation: Follow step-by-step on komoot, or download GPX file

About the route

Not far from the A4212 in Eryri (Snowdonia), Cwm Prysor Viaduct spans a lush valley with all the drama of a Highlands rail bridge — and none of the crowds. Built in the 1880s, this nine-arch structure once carried trains between Bala and Blaenau Ffestiniog. Today, it’s part of a scenic walk along a disused railway line where nature has reclaimed the trackbed, giving the trail a wonderfully wild, secluded feel.

If you’ve ever caught a fleeting glimpse of a dramatic rail bridge from the A4212 between Capel Celyn and Trawsfynydd, you’ve probably spotted Cwm Prysor Viaduct — a graceful relic of Victorian engineering. The old Bala and Festiniog Railway once ran through here, and though the trains are long gone, nature has quietly reclaimed the trackbed, turning this into one of Snowdonia’s most atmospheric hidden walks.

You can make this a quick detour or a full linear adventure along the disused line, soaking up views, viaducts, and moody hillsides along the way.

Distance: 1 mile (to the viaduct and back)
Extended route: ~8.8 miles (out-and-back to road near Trawsfynydd)
Time: 1–4 hours depending on route
Start/End Point: Lay-by on A4212 near Llyn Tryweryn
OS Grid Ref: SH 81438 36262
What3words: stack.crouching.scramble
Dog friendly: Yes 🐾
Public transport: None direct — nearest stop is Trawsfynydd

Route difficulty

The trail is relatively flat and easy to follow, but the mud can be a real limiter. Even in dry spells, the railway trackbed holds water like a sponge — so bring sturdy footwear and consider turning back after the viaduct if it’s been raining heavily. That said, the terrain itself is straightforward, and there’s little elevation gain.

Walk type: Easy to Moderate — mostly flat and steady, with no real incline but very boggy in places.
Terrain: Disused railway path, forestry track, and deeply puddled muddy ruts.
Accessibility: Not suitable for wheelchairs or pushchairs — path is uneven and frequently waterlogged.
Dog-friendly: Yes — dogs are welcome on leads, but be aware of sheep along the route and limited visibility in spots.

Why you’ll love this walk

Rarely visited viaduct with huge views
A “path in the sky” reclaimed by nature
Tadpoles, newts, sheep, and solitude
Extendable into a much longer railway hike

Route highlights

From the lay-by to the viaduct

Park in the lay-by just west of Llyn Tryweryn, then hop over the ladder stile to join a forestry road. At the first fork, go left — this is the start of the disused railway. You’ll follow two muddy tyre tracks through a landscape slowly being reclaimed by nature. Tadpoles squirm in puddles, the old cuttings guide your way, and before long, you’ll catch a glimpse of the viaduct.

Cwm Prysor Viaduct

Spanning 450ft across the valley and rising 120ft above the ground, the viaduct is a stunning reminder of the area’s industrial past. Its nine arches feel almost Roman in scale, and the deck (a grass path now) is open for walking thanks to a permissive access agreement. There are spots just before the bridge to view the arches — take care, as the terrain can be uneven.

Beyond the viaduct (optional)

If the conditions are dry and your boots are up for it, continue on. The route hugs the hillside and passes through weathered woodlands and pasture, with panoramic views toward Arenig Mawr. Eventually, the trail meets a single-track road near Trawsfynydd — around 4.4 miles from the start point (one way).

Things to do nearby

Llyn Celyn: Scenic reservoir with a controversial backstory
Arenig Mawr: A quieter Snowdonia summit
Trawsfynydd Lake: Ideal for picnics or extending your walk
Bala: A solid base with cafes and mountain shops

Getting to the starting point

The start is a small lay-by on the A4212. From Capel Celyn, drive west past Llyn Tryweryn and watch for the pull-in on your right. From Trawsfynydd, climb the hill — the lay-by is near the crest. The entrance is a little hidden but marked by a gate and stile.

✔️ Good to knows

This is a permissive path, not a formal right of way
Very limited parking
No signage — use OS Maps or a GPS app
Often muddy — especially beyond the viaduct
Keep dogs under control (lots of sheep)
Public transport won’t get you close

🧭 Following the route

I use komoot and OS Maps for offline navigation. To follow the full walk step-by-step on komoot, simply click this link here or download the GPX file and upload it to your chosen app, such as OS Maps.

Cwm Prysor Viaduct: an atmospheric walk in Eryri