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Connecting Cities | High Speed 2

High speed rail

Improvements in urban connectivity are impossible without a step-change in rail network capacity.

Despite having largely completed its journey through Parliament and with construction having started in earnest at Euston station, High Speed 2 (HS2) continues to stir up debate. Why is it being prioritised over other corridors such as those in the North or South West? Does it follow the best route? Is it necessary at all?

The UK rail network is the busiest in Europe, even before you consider freight. On key routes, existing lines are already reaching capacity.

With rail’s modal share having doubled since the 1990s, it is small wonder that it is now creaking under the strain.

Passenger growth

Significant passenger growth would result in a dramatic drop in reliability and a disproportionate hike in operating costs.

To facilitate even a modest modal shift in favour of the train, we need a step-change in the railway’s capacity to move people and things around nationally.

People often talk about the possibility of upgrading existing lines with new signalling, improved line speeds or even extra tracks.

While these may be useful interventions in isolation, none can provide a significant regional or national improvement in capacity.

Major upgrades to the existing infrastructure are excruciatingly complex. To say that it is difficult to accurately estimate costs and timescales would be an understatement.



The highest capacity lines are the ones that operate the rolling stock with the closest performance parameters – acceleration, deceleration and top speed. If the difference between top speeds is minimised, then the trains can get nice and close to one another and you can run more of them per hour.

The trouble is, the British rail network represents the opposite of this situation.

Slow freight and stopping trains intermingle with regional medium-speed services. Trying to negotiate a way through this traffic is the most effective capacity-killer of all.

High speeds and limited stops mean that high speed trains require a large headway, with other services being pushed out of the way. Route capacity suffers.

High speed segregation

The answer is to extricate these fast trains from the rest of the railway: in other words, the segregation of long distance, high speed services.

Major areas of population and economic output as a series of urban “hubs” are the clusters of cities that are connected by less than an hour’s travel and have a population greater than 1M people.

When thinking of a rail network, we can consider connectivity on two levels: intra-hub – travel within hubs – and inter-hub – travel between hubs. Combining these two levels of connectivity gives us a map that starts looking like the current UK railway network.

It is easy to see that there is a conflict between intra-hub and inter-hub services through the Midlands and the North.

At the same time, the north-south corridor gets busier as you get closer to London, with the southern-most section carrying all north-south inter-hub traffic from the Midlands, the North and Scotland.

Inter-hub connections

Inter-hub routes from London to the south coast and from London to Bristol and South Wales do not carry traffic from other hubs. They also do not have to carry intra-hub and inter-hub traffic at the same time. These corridors are therefore less critical to overall capacity.

High Speed 2’s route aims to solve intra-hub and inter-hub capacity constraints between the North, the Midlands and London by providing a new line carrying high speed passenger trains on the north-south rail corridor.

By segregating long distance, high speed services onto their own lines, capacity is created for more intra-hub services, as well as freight and local trains. The existing urban and regional networks then feed passengers onto HS2.

In network science this represents a “hub/rich club” model, where high flow rates exist between hubs that are themselves fed by lots of interconnecting pathways.

HS2 phase 1

HS2 Phase 1 covers the southernmost corridor that carries all inter-hub traffic north of London. This section will have the greatest capacity, comprising what is effectively a straight line from London to Birmingham. HS2 Phase 2 provides two “legs” to separate intra-hub/inter-hub services across the North and inter-hub traffic from Scotland.

But why must it be high speed?

The reason for constructing this new line to high speed specifications is simple: to incentivise the diversion of long distance, high speed services from the three major north-south main lines – the West Coast Main Line, the Midland Main Line and the East Coast Main Line – onto the new segregated line, journey times from the northernmost destinations must be at least as good as the current service.

Essentially, High Speed 2 gives us six lines for the price of two.

High speed rail isn’t really about high speed at all.

Oddly enough, high speed passenger services only form a small part of what the UK’s new high speed rail line will deliver.

In combination with upgrades to intra-hub connectivity, it is the freed capacity on the rest of the network that will have the greatest benefit for passenger and freight transport.

Yet the story doesn’t seem to be getting through. As engineers, it is not just our duty to create new infrastructure, it is also our duty to communicate its purpose. Helping the British public to get behind HS2 must be a priority for our industry if we are to truly make this megaproject a success.

  • Gareth Dennis is an engineer and writer, specialising in railway systems. As well as roles in engineering consultancy, he leads the local section of his professional institution and is a lecturer in track systems at the National College for High Speed Rail. Follow him on Twitter: @garethdennis

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Readers' comments (1)

  • A few points on the narrative and diagrams included in the article:

    - Diagrams include the Midlands as part of the same "hub" area as Transpennine and Merseyside. The geographic separation between the Midlands and the more northern areas suggest that this is misleading.

    - Inter hub diagram shows no connectivity between Midlands/North England and the South West. This is a significant omission.

    - No mention is made of the importance of planning intra-hub train services to have good connectivity with inter-hub services. Without this journeys by train are less attractive to prospective passengers. The way HS2 has been planned at Birmingham with no direct connection between local train services (centred on New Street and HS2 services (at Curzon Street) is a major short-coming of the HS2 plans.

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