Metro-style rail on the Core Valley Lines

Passengers in south-east Wales and beyond will soon benefit from new rolling stock, increased service frequency, and the large-scale upgrade of the Core Valley network. Chris Howe reports.

In this article:

• Extensive Overhaul: Since 2020, Amey Infrastructure Wales has been modernizing the Core Valley rail network to deliver a metro-style service.
• Electrification & Upgrades: The £1 billion project includes electrification of 170 km of track, new rolling stock, and infrastructure upgrades.
• Transformational Impact: Promises increased service frequency, improved access for nearly half of Wales’ population, and expanded capacity.

For the past four years (work started in August 2020), Amey Infrastructure Wales (AIW) has been delivering a major overhaul of the railways in south-east Wales, with the aim of delivering a metro-style rail service on the Core Valley (CV) lines.

This will require much more than electrification, with track and signalling being upgraded and new rolling stock introduced to allow for a significant uplift in service frequency.

The original budget for the upgrade was estimated at £734 million. However, in March 2023, it was announced that the budget had increased to £1 billion owing to a significant rise in inflation and delays attributed to the pandemic.

But even this higher cost seems modest when you consider what the upgrade promises to deliver.

The word ‘transformational’ is often overused, but South Wales Metro genuinely does have the potential to transform rail travel across south-east Wales, by improving access to rail services for almost half of the total population of Wales.

The key headline figure, which will deliver the largest part of the benefits, is the 170 single-track kilometres (105 miles) of electrification. This will allow Transport for Wales to introduce new rolling stock and increase the service frequency, which in turn will boost capacity.

The project is being delivered by AIW, which also manages the CV lines and will continue to manage the network once the upgrade is complete.

In March 2020, ownership of the CV lines passed from Network Rail to Transport for Wales, which in turn leased the assets to AIW, which is responsible for operation and maintenance. TfW assumed control of passenger services in February 2021 and will continue to manage day-to-day operation of railway services.

The Core Valley network comprises five primary routes - the Aberdare, Coryton, Merthyr Tydfil, Rhymney and Treherbert lines to Cardiff Queen Street. The CV lines also incorporate the Cardiff City line to Ninian Park and the Butetown branch line, otherwise known as the Cardiff Bay line (currently) to Cardiff Bay station.

The new bi-mode and tri-mode multiple units (which have superior acceleration compared with the Class 150 diesel multiple units that currently operate on those routes), combined with signalling upgrades, will provide the headway required to (in some cases) double the number of trains operating on the CV lines.

Most of the lines are already reasonably well -served, with a two-trains-per-hour service during the week on most of the Valley routes. However, once the Metro upgrade is complete, this will increase to a daytime 4tph service during the week on most routes.

Coryton will retain the existing 2tph frequency. However, passengers will benefit from the additional capacity provided by the new three- and four-car Class 756 tri-mode units (passengers on the Coryton line have benefited from the introduction of four-car Class 231 DMUs since their introduction on the route last year). The ‘231s’ will eventually move to the Ebbw Vale line and operate between Cheltenham Spa and Maesteg.

In RAIL 957, I explained why the rolling stock procurement for TfW seemed particularly complex. But in essence, the requirements for each line are sufficiently dissimilar that an individual approach was required when deciding which units to order.

Recent attempts to electrify the UK rail network have been particularly challenging, which has so far meant that electrification (particularly in England) has been expensive.

This seemingly has created a barrier that has curbed any attempts at a rolling programme of electrification.

In terms of physical challenges, electrifying the CV lines has been no different, given the topography, with existing road bridges, structures, tunnels and listed structures.

The challenges of electrifying the CV lines and a lack of electrification beyond the core network are key reasons why several rolling stock solutions were decided upon.

This approach has allowed AIW to design a solution that utilises discontinuous electrification, which has helped to keep the per kilometre costs for electrification comparatively low - at least in terms of the UK.

To that end, the CV lines will be served by two distinct types of trains, with Class 398 Citylink units (which are bi-mode) operating from Treherbert, Aberdare and Merthyr Tydfil into Cardiff. These units can take power from overhead 25kV lines and are also fitted with batteries, so can operate on short non-wired sections.

The Coryton and Rhymney lines will be served by Class 756 tri-mode multiple units, which can also draw power from overhead lines, but are fitted with diesel engines in addition to batteries.

Being bi-mode and tri-mode means that both unit types can operate on sections without wires, with the Class 756s able to operate on longer non-wired sections, such as the Vale of Glamorgan line from Cardiff Central to Bridgend.

The Class 398 tram-trains will only be required to operate for relatively short distances without wires, such as on sections under existing bridges which could not be easily adjusted to allow for the overhead line equipment (OLE). As such, these units only require batteries.

Although discontinuous electrification may not be considered optimal, it does mean that the benefits of electrification can be delivered cost-effectively, which ultimately meant that the upgrade was considered affordable and ultimately deliverable.

Pontypridd station is a prime example of a situation where it would not have been straightforward to install wires. The station’s Grade 2 Listed status means that significant adjustments couldn’t be made to the station structure or the footbridge, therefore OLE will not be installed at the station.

The most significant section of discontinuous electrification is between Lisvane & Thornhill and Caerphilly, on the Rhymney line. This section, which passes through the 1.8km (1.1-mile)-long Caerphilly Tunnel, will not be wired owing to the constrained tunnel bore.

The non-electrified sections will take one of two forms depending on the clearance available.

Some sections will not have OLE at all, and as such the pantograph will have to be lowered and raised at certain points before entering one of these sections.

Raising and lowering of the pantograph will be initiated automatically when the train detects a signal from trackside RFID (Radio Frequency Identification) devices. The signals will also automatically tell the units when to switch between battery and overhead power, and vice-versa.

Other sections, known as Permanently Earthed Sections (PES), will have wires, but they will be neutral.

This allows for the contact wire to be positioned higher or lower than standard, and is being used where clearance between the wire and bridge or structure is not sufficient for a live section.

Using neutral sections has the benefit of allowing the pantograph to remain in the raised position while passing through the section.

Work to electrify the Aberdare, Merthyr Tydfil and Treherbert lines is largely complete, with the final 16km (10-mile) stretch between Pontypridd and Treherbert station energised in June. Before that, the first electric test operated between Cardiff and Pontypridd in April.

Once the upgrade is fully complete, the three routes will be served by Class 398s, which being tram-trains have the capability to operate along on-street sections.

However, these routes will initially be served by Class 756s until later next year, once the depot at Taffs Well is completed and the Citylink units are ready to enter service.

Once the Class 398s have been introduced, the Class 756s will move to the Coryton and Rhymney lines and will operate along the Vale of Glamorgan line to Penarth, Barry Island and Bridgend.

To allow for the upgrade, which will enable up to 18 trains per hour to call at Cardiff Queen Street in both directions, the track and signalling has been upgraded and modernised.

The work to upgrade the station, track and signalling at Queen Street began in February this year, with further work taking place during the August Bank Holiday.

The upgrade of the Treherbert, Aberdare and Merthyr Tydfil lines is largely complete. However, work is still ongoing on parts of the route - most notably at Taffs Well station, where work was still under way in August to complete the new accessible overbridge, car park and drop-off facilities.

Work is also continuing on the new depot and Integrated Control Centre (ICC) adjacent to the railway station.

The new £100m depot and ICC will be home to the 36 Class 398s, as well as over 400 train crew, 35 maintenance staff, and 50 ICC staff. The connection from the main line to the depot was installed in February 2024, with the first tram-trains arriving at the end of March.

Introduction of the Class 756s has enabled TfW to bring forward some of the benefits of the Metro, prior to the introduction of the ‘398s’. It has also allowed TfW to put to work the 24 FLIRT units, which despite having started to arrive at the end of 2022, only began daytime testing on the CV lines in April 2024.

Work on the Coryton and Rhymney lines is at a less advanced stage.

However, installation of the masts and overhead wires continues at pace, and wires on the Coryton line will reportedly go live imminently. It is hoped the upgrade to Rhymney will be fully completed by autumn 2025.

Although no wires will be installed between Lisvane & Thornhill and Caerphilly, a conductor bar has been installed at Platform 1 at Caerphilly station, to allow for stationary charging of Class 756 batteries. The masts for the short conductor bar section were installed in early August, with work to fit insulators taking place throughout the same month.

The rollout of the Class 398s and electrification of the Rhymney and Coryton lines will enable the introduction of the Class 756s on those lines, which in turn will allow for Class 231s between Cheltenham Spa and Bridgend and from Ebbw Vale to Cardiff Central.

The £1bn funding envelope for the Core Valley lines upgrade does not extend to the Vale of Glamorgan line, which remains the property of Network Rail.

However, new units on the Glamorgan line will benefit passengers between Cardiff Central, Penarth, Barry Island and Bridgend.

It is proposed that Barry could be served by as many as five trains per hour in each direction from Cardiff Central, with three trains per hour heading to Barry Island, while two trains per hour will continue to Bridgend.

Once the new Citylink units have been introduced, it is proposed that two trains per hour from Aberdare, Merthyr Tydfil and Treherbert will call at Cardiff Bay and at the new Butetown station which is currently under construction.

As part of the upgrade, Cardiff Bay station is receiving a new platform, signage and passenger information screens.

This upgrade also includes passive provision to extend the line south in future.

The upgrade and new station form part of what is hoped will be a stepping stone towards what TfW is calling ‘Cardiff Crossrail’.

This £100m scheme will result in new platforms being constructed at Cardiff Central, along with a connection to the Cardiff Bay line. Plans also include the addition of a third platform at Cardiff Bay station and extension of the line to a new station to be constructed at Pierhead Street.

The new link would entail the Class 398s operating on a short on-street section from new platforms to be constructed at Cardiff Central (in the car park on the southern side of the station). From there, the line would cross through Callaghan Square, joining the Cardiff Bay line close to the junction between Callaghan Square and Bute Street.

The full business case is expected to be finalised for the scheme by the beginning of this year, after which TfW will need to submit a Transport and Works Act Order (TWAO) application. If permission is granted by 2026, construction of the new platforms, connection and extension could be completed by 2029.

Many promises were made in 2018 when KeolisAmey won the bid to operate the Wales and Borders franchise, with new rolling stock and the overhaul of the south-east Wales rail network on the horizon.

But the intervening years have been extremely challenging, with the pandemic affecting passenger numbers and inflation pushing up costs.

There have also been setbacks with rolling stock either not being delivered on time or suffering reliability issues when in service. Then there have been operational and maintenance issues, leading to widespread cancellations across the network.

However, after six years, it seems that passengers in Wales and beyond have cause to be optimistic about Wales and Borders services going forward, especially those passengers who live in south-east Wales who will soon benefit from new rolling stock and the large-scale upgrade of the Core Valley network.

About the author: Contributing writer Chris Howe is a freelance transport journalist with special interest in transport civil engineering projects, and a prolific pro-rail campaigner.

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