The primary purpose of the line was to carry slate from the Ffestiniog quarries to a specially built quay at Deganwy (occasionally referred to as St George's Dock) for export by sea.

Although only a little over 27 miles (43 km) between Llandudno and Blaenau Ffestiniog, the journey takes over one hour, largely due to the sinuous and steeply graded nature of the route taken (particularly south of Betws-y-Coed).

The line's proximity to the River Conwy at its northern end has led to periodic problems with flooding over the years.

[7] The line was again closed from 27 December 2015 for seven weeks, after the formation was damaged by floodwater in more than 100 places following heavy rain on Boxing Day.

[11] Storm Ciara's heavy rainfall on 8–9 February 2020 once again damaged the trackbed near the Conwy estuary north of Llanrwst, closing the line yet again.

The line's summit 790 feet (240 m) above sea level is located midway through the tunnel and the gradients either side of it are as steep as 1-in-47 (2.1%) on the southbound ascent through Pont-y-pant and 1-in-43 (2.3%) on the climb out of the terminus at Blaenau for northbound trains.

[17] The line is currently served by 6 trains per day in both directions from Monday to Saturday between Llandudno and Blaenau Ffestiniog (and vice versa), with the first two southbound services of the day starting (and first northbound service turning back) at Llandudno Junction and terminating in Blaenau Ffestiniog.

A feature of the service is the availability on Conwy Valley trains as well as on local buses in Snowdonia of the new "Tocyn Taith" day ticket.

[22] 1998 the first steam working in the preservation era was to run back to Blaenau Ffestiniog from London behind a BR Standard 4 Tank no 80079.

The first attempt up the route on 2 May 1998 ended in failure as the engine stalled near Pont-y-Pant and due to the water supply running low resulted in the train having to be rescued and then returned to Llandudno.

A second attempt a day later, this time with just four coaches instead of the previous six, proved successful, with 80079 becoming the first standard gauge steam locomotive in Blaenau for 31 years.

[23] Five months later on 17 October the BR Standard tank no 80079 returned for a rematch on the route, this time she was to haul four coaches instead of six, and she was working the trip with another member of her class no 80098.

Following repair work, the line from Llandudno Junction to North Llanwrst was re-opened on 24 July 2019; the rest of the route would reopen in August.

To mark the full reopening of the line, Transport for Wales Rail in co-operation with West Coast Railways ran a train called "The Conwy Quest" from Chester to Blaenau Ffestiniog via Llandudno Junction on Saturday 3 August.

Locos that have visited the route in recent years include: 44932, 45231 The Sherwood Forester, 45407 The Lancashire Fusilier, 45690 Leander, 46115 Scots Guardsman,[27] 48151[28] and 61994 The Great Marquess.

Beyond the new station, the line was used only for goods traffic connected with Trawsfynydd, although occasional special passenger trains have been run at times.