HomeInfrastructurePorthmadog bypass & its four rail lines

Porthmadog bypass & its four rail lines

Listen to this article

The Welsh Government placed a £35 million contract in December 2009 for the construction of 5.3km of new road taking the A487 around the towns of Porthmadog, Minffordd and Tremadog in North Wales.

A joint venture (JV) partnership of Balfour Beatty and Jones Bros Civil Engineering UK was asked to construct a 7.3m wide carriageway on the flood plain of the River Glaslyn to the north of the Cambrian Coast mainline railway.

Along with sub-contractors Balfour Beatty Rail and Cleveland Bridge UK, the JV has built 8 new bridges, a number of smaller structures and a considerable length of embankment to create the substructure for the new road.

450,000m³ of fill, weighing about 900,000 tonnes has been used on the project. It was sourced from the adjacent Minffordd Quarry and transported direct to site by a network of haul routes. As a result, 106,000 road wagon movements were avoided on the existing highway network.

Four railways

Four different railway lines have been affected by the work. The obvious one is Network Rail’s Cambrian Coast line, the line of the bypass running close to it over much of its length. Also involved are the Ffestiniog Railway (FR), the Welsh Highland Railway (WHR) and the Welsh Highland Heritage Railway (WHHR). The FR is well known.

The WHR is the relatively newly restored and reopened heritage line operated by the FR. It runs from Caernarfon on the route of the former Welsh Highland line for most of its length, but deviates from it at the Porthmadog end to join into the Ffestiniog Railway’s Porthmadog Station. The WHHR is a separate, shorter heritage railway that operates on the original line of the Welsh Highland through the Porthmadog area.

The Cambrian Line was the most affected by the new bypass. 850m had to be realigned by up to 25m onto a new embankment built alongside to make way for the new road. A new 3-span viaduct carrying the bypass across the River Glaslyn is only about 50m upstream of the railway river bridge.

In addition, a bailey bridge was built over the river above the new viaduct site to carry the haul road. Where the road and rail routes run side-by-side, new bridges and culverts were built beneath the road connecting with existing equivalents under the rail line.

Finally, a level crossing providing the existing road access into Minffordd Quarry became redundant as the quarry will in future be accessed from the new bypass.

Shorter route

The realignment necessitated first the construction of a new rail embankment, then the laying of new track onto it. Track installation was carried out by Balfour Beatty Rail. On 7/8 May this year the line was closed and bus substitution services took over whilst the existing line was cut and slewed over at each end of the new embankment and connected to the new tracks. This raised an unexpected issue, for the JV at least.

The Cambrian Line is the site of Network Rail’s ERTMS trial, and the new railway is actually about 7m shorter than the old alignment. Network Rail’s signalling team recognised the significance of this and updated the ERTMS system software for the route before the line was reopened after the diversion.


At Minffordd Quarry the old level crossing was removed, since, as already mentioned, access to the quarry was to be diverted via the new bypass.

Nearby, a small drainage culvert passing under the original railway route had to be replaced by one under both new road and rail embankments. However, the environmental requirements for the site dictated that a large structure be built with sufficient headroom for bats to fly from one side to the other, so what was originally a culvert a few hundred millimetres in diameter has now become a significant structure.

Apparently this is not the only such example on the route of the road, and some culverts are “almost high enough for a double-deck bus”!

Earthworks operation. Photo: the rail engineer.

Hydraulic modelling

The Cambrian Line was also affected by construction works and plant operations in other ways. At the river crossing, construction upstream of the piers of the new road bridge and of the temporary bailey bridge could have had an effect on the river bed and the foundations of the rail bridge.

Network Rail insisted upon hydraulic modelling before work began, and regular monitoring of the river bed and rail bridge foundations throughout the works and after completion, to ensure that there was no risk to their structure. Given the Glanrhydd accident in the 1980s when 4 people died as the result of the scouring away of the foundations of a rail bridge, their concern is understandable.

One of the largest crawler cranes in Europe was brought in for the construction of the permanent river bridges. This 630t monster was quite large enough to affect the adjoining railway should there be any mishap during its use, and so detailed methods of working were agreed with Network Rail beforehand.

The same procedure was adopted elsewhere when cranes or similar plant were to be used where they might affect rail safety.

Ffestiniog Railway

A new bridge carries the Ffestiniog Railway over the new bypass. It was constructed alongside the line on temporary trestles by Cleveland Bridge. In January 2011 the JV was given a 4 day line closure by FR during which they built the concrete abutments of the new bridge and dug out the “plug” of soil from between them.

On 7 February the deck was slid into place on the abutments and within a week the complete structure was handed back to FR. Track was relayed over the bridge while additional works were carried out on either side, including the remodelling of Minffordd Station and double-tracking through it. The first train crossed the new bridge on 2 March, only 6 weeks after the line was closed.

The FR gave the JV a great deal of co-operation, including an agreement for a temporary level crossing at Minffordd. Manned by a JV employee who was trained as a crossing keeper by the FR, this plant crossing allowed the removal of 10,000m3 of material, cut out of the existing ground due to the differential level between the rail and new road levels, without the use of any public highways, a great benefit to the local population.

Welsh Highland

In contrast, the new road crosses over the route of both the WHR and WHHR. A site for the new bridge was chosen where the 2 lines lie close together. The structure is a 3-span design, also fabricated and erected by Cleveland Bridge, while its concrete substructure was built by the JV.

The two railways pass through the bridge’s centre span, the side spans allowing access for both the local landowner and for future highway maintenance. A temporary level crossing over both rail routes was established at the site and the WHHR insisted on a different approach here from that taken by the FR at Minffordd as the line was to remain open during the works.

A WHHR employee was used as crossing keeper, and gates, traffic lights and a telephone were all provided for his use. A concrete slab formed the crossing surface, and one of the duties of the crossing keeper was to ensure that the flangeways in this were kept clear for the passage of trains.

Environmental matters

The Environment Agency was heavily involved in the project, especially regarding the Glaslyn river crossing, and so were other environmental organisations, archaeologists and wildlife organisations. All placed constraints and requirements upon the construction team with provisions being made for the protection of reptiles, bats, badgers and birds.

Virtually the whole of the site lies in the flood plain of the river and this too also caused complications. All these issues, and the need to liaise with the four separate railway companies involved, required careful and detailed pre-construction work planning and agreements.

View of both the bypass and Cambrian Line from Minffordd quarry. Photo: the rail engineer.

Local involvement

Both Balfour Beatty and Jones Bros are rightly proud of their liaison with the public throughout the contract. They have held regular meetings with local councillors to give them advance notice of works and established a visitor centre close to their own offices.

This provides information about current works and future plans and is open every working day. Even a large print of the route map with appropriate captions placed in the window of a local Tesco has proved popular.

The project team has supported local charities and allowed the mountain rescue team to use the visitor centre in the evenings as a training room. They have consulted local stakeholders about the appearance of the structures on the road, and have sought the views of the relevant railway companies about the colour and appearance of the new rail bridges.

When, in response to a number of accidents and incidents across the railway network, there was a local campaign about level crossing safety, the contractors joined with the FR to work with IoSH and a local organisation called “Working Well Together” on a two day event. It included a practical demonstration on FR premises which highlighted the real danger to people of misusing a level crossing.

The new Porthmadog, Minffordd and Tremadog bypass is due to open in December 2011. On completion, it will represent a true joint venture between the two main contractors and four railway companies. There aren’t many projects like it….

Chris Parker
Chris Parkerhttp://therailengineer.com

Conventional and slab-track, permanent way, earthworks and embankments, road-rail plant

Chris Parker has worked in the rail industry since 1972, beginning with British Rail in the civil engineering department in Birmingham and ending his full-time employment at Network Rail HQ in London in 2004. In between, he worked in various locations including Nottingham, Swindon, Derby and York.

His BR experience covered track and structures, design and maintenance, followed by a move into infrastructure management. During the rail privatisation process he was a project manager setting up the Midlands Zone of Railtrack, becoming Zone Civil Engineer before moving into Railtrack HQ in London.

Under Network Rail, he became Track Maintenance Engineer, representing his company and the UK at the UIC and CEN, dealing with international standards for track and interoperability, making full use of his spoken French skills.

Chris is active in the ICE and PWI. He started writing for Rail Engineer in 2006, and also writes for the PWI Journal and other organisations.


  1. Do contractors need accommodation, on a room only basis mon to fri please call Beth on 07855 790112 for details and prices at Castell, 8 Garth Terrace, Porthmadog.

  2. A small but important correction: The WWHR (Welsh Highland Heritage Railway) is not the original line of the Welsh Highland Railway as stated in the ‘Four Railways’ section. 

    The history of the Welsh Highland Railway in preservation is somewhat confusing and has been fraught with difficulties.  It is not sensible or politick of me to explain all this here, but it is worth pointing out that the WHHR started out as the Welsh Highland Railway (1964) Ltd. with the intension of rebuilding the complete line as we see it today.  However, the entire length of their line is on the site of standard gauge sidings formerly used to transfer slate onto the Cambrian Coast railway.

    Meanwhile, the Ffestiniog-owned Welsh Highland Railway of today uses a cross-town route into Porthmadog station that is close to the route taken by the original Welsh Highland Railway.  However, it deviates in order to avoid running down residential roads, involving the construction of an angled connection onto Britannia Bridge.  The original Welsh Highland Railway of old had an intermediate station along this route, but I think it is sensible to concentrate on running trains into the harbour station.  From the point of view of today’s visitors, it would be sensible if a platform were provided at Pen-Y-Mont to allow people to interchange between the two railways.  Not that this has anything to do with the bypass per se, but it is all relevant to transport and tourism – half the reason why the new road is being built.


Please enter your comment!
Please enter your name here

This site uses Akismet to reduce spam. Learn how your comment data is processed.