How do you build a new high speed railway, anyway?

A day out on HS2.

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The team from HS2 are very nice about the fact I hadn’t noticed their massive new overpass. “Of course, you’ll have seen this from the train,” they say, as we drive beneath the Small Dean Viaduct, one day to speed trains across the A413, a local road and the existing Chiltern rail line. But despite the fact it’s 345 metres long and weighs 4,500 tonnes, I’d missed it, because I’d been looking out of the wrong window or possibly at my phone. Oh well. At least I get to see it now. It’s pretty good. Top viaduct. Would pass under again.

Every now and again, someone is nice enough to invite me to go and look at a half-finished construction scheme somewhere, so I can marvel at the cleverness of a modern engineering project. In the past few years I’ve had a go on Cecilia, the Tunnel Boring Machine then gnawing its way under the Chilterns; I’ve been down in a waterlogged hole in the ground that will one day house Old Oak Common station; and I got a sneak preview of the Elizabeth Line, a trip that resulted in a grumpy DM from a current Cabinet minister asking how the bloody hell I’d got onto that mailing list and how he might do the same. Today – just over five months ago, actually, but let’s not be picky – I’m off to a relatively rural stretch of Buckinghamshire to look at some more megastructures that will one day be part of HS2.

When I’d told people I was doing this, some expressed surprise, of the “Oh, are they still building that?” variety. The last government did scrap everything north of Birmingham, it’s true, with abysmal consequences for service patterns, value for money and faith in the ability of this country to do literally anything all at the same time. Actually, the mad idea of terminating trains at Old Oak Common, rather than Euston, has thankfully been dropped – the tunnel boring machines began their journey east early this year – and the stretch of the line from the western outskirts of London to the eastern ones of Birmingham is already well under way.

But this, as I learn in a temporary site office near Wendover – its walls festooned with signs explaining safety procedures, warning me not to drink the water or, more confusingly, outlining the symptoms of prostate cancer – that doesn’t mean it’s been easy. For one thing, any route between the two cities means a construction site dozens of miles long but only a few metres wide, a logistical nightmare. It also inevitably involves crossing the Chilterns, a chalk escarpment running southwest-northeast from southern Oxfordshire to Luton: hills and trains, you’ll be aware, are not good friends.

Building a rail line through a hilly area of outstanding natural beauty would be quite difficult enough – but this is plush London commuter territory, so the planning constraints imposed by the High Speed Rail (London-West Midlands) Act 2017 (HS2 Act) have complicated things further. Even after the scheme as a whole was agreed, HS2 Ltd and its subcontractors needed to get final approval from local planning authorities for the design of its various structures. It also employs agricultural liaison officers, to manage access to and impact on the land the route passes through, and deal with compensation and complaints.

Put these topographical and planning constraints together, and EKFB – the joint venture building the 80km section of the line between the Chiltern tunnel and Long Itchington Wood in Warwickshire – has found itself responsible for building 15 new viaducts and 7km of tunnels. To keep noise to a minimum, parts of the line will also be in either “cuttings” (that is, below ground level), or “false cuttings” (which are at ground level but disguised by earthworks). In an attempt to keep locals onside, a political decision was taken to ensure that no existing road will be severed, no matter how small: that means 81 bridges, too.

(On this occasion, I’m not going to dwell on the question of whether this is the best way of doing things, or whether it might in fact just be running up the costs and extending construction time in a doomed attempt to win support from people who are inevitably going to be annoyed by disruption and construction traffic. The contractors I’m meeting today say their job is simply to deliver on the project as defined by the government, which is fair enough. So today, I’m here to learn about the practical side of things.)

All this requires moving an enormous quantity of earth before anyone can even think about putting down rails. The path of the scheme is currently traced by an 80km service road, to move earth from sites that have too much of it to those that have too little. (Most of the “haul road” will be removed, but in a few places the council has asked for it to be retained permanently.) Elsewhere, temporary conveyor belts have been constructed to minimise HGV traffic, too.

The logistical challenges don’t stop there. Chalk gets weaker when wet: heavy construction traffic thus can’t move in the winter months without destroying the landscape, and a wet summer could cause unexpected delays. (Last year, thankfully, was dry.) Temporary ponds have to be built, to collect water runoff and ensure it doesn’t soak the local fields in construction pollution. And because different stretches are being built by different subcontractors, there are a lot of meetings: about matching designs, managing physical interfaces, ensuring contractors aren’t banging into each other.¹

That’s enough desk learning, let’s go and look at what this means on the ground.

Note the gaps.

First we troop through the mud, following the line of the half finished Wendover Viaduct, its deck built in the fashionable “weathering steel” that rusts slightly when exposed to the elements, both protecting its surface and leaving it a colour somehow reminiscent of chocolate orange. On one side graze some sheep; on the other stands a brand new house, occupied by the farmer responsible for them. (He decided to stay on his family’s land, but used his compensation payment to upgrade, apparently.) At the access point to the viaduct itself, we find a booth, occupied by a single young man named Jack – everyone else is at Christmas lunch – who gives us our safety briefing, and explains, just in case we were wondering, where the defibrillator is.

Then we climb to the viaduct proper, and are suddenly a dozen metres above the rolling fields. It’s not a bad view, though it won’t be available to passengers once the line is finished: once complete, the new viaduct will be bounded by noise dampeners and a concrete wall, and will, in any case, take mere seconds for the high speed trains to cross.

How, you are wondering, do you build something like this? The nine concrete piers supporting the viaduct, made offsite in Northern Ireland, are put in position first; then the precast steel spans made in France pushed out from the more finished end, and lowered onto the supports. That, though, only amounts to about half the width of the final structure: on top of them will sit a deck, made of rebar (metal mesh, essentially), then covered in concrete.

Here comes the clever bit: the concrete is poured by a thing known as “the traveller”. Despite the vaguely mysterious name, this is effectively, well, a massive mould for pouring concrete into: its full name, “cantilever formwork traveller”, comes from the fact that, after pouring concrete to make one section, it moves along to the next section but one (it skips them to balance the weight, then comes back to fill in the gaps later).

Moving the traveller is a big deal: it takes two days to move it 50 metres, in two bursts, and at each point it has to be comprehensively secured by massive cables. It can also only move once the concrete has reached a particular strength: that means it works more slowly at this time of year than it would in warmer months because it takes longer to set in the cold. (When concrete gets too warm, by contrast, there’s a danger it cracks as it dries – not what you want if you’re going to run trains over it – so the team has to water it.)

The last bit of the structure will be the “ears”, the very tips of the metal piers. These give the reassuring impression they’re supporting the concrete viaduct, but really they’re just decorative. They have to go on last because otherwise the traveller would bang into them.

Before returning to ground level, we take a second to look inside the viaduct itself (making it hollow makes it, unexpectedly, stronger). Once the structure is finished this will not be possible, except for the odd maintenance trip.

It’s only then – faced with what looks like a metal corridor, stretching a quarter mile into the distance, and feeling like I’m on the set of a scifi movie – that I realise quite how big this thing is.

We leave the site via a road under a red and white square arch surrounded by blue traffic cones – there, apparently, to remind HGV drivers that they’re going under electrical cables – and drive a mile or two up the road to pay a quick visit to the Wendover Green tunnel. The word “green” here doesn’t mean “environmentally friendly”, except in so far as rail generally is: it means it’s being built on the surface and then buried with grass and other plants, so that it disappears into the landscape.

Inside the tunnel it is raining. This is nothing to worry about, I’m told: it’s not been waterproofed, yet.

The new tunnel is being built from a sort of giant concrete lego set, consisting of five different sorts of piece (one for each side, two to make the roof and a middle section dividing the two tracks), lowered into place and then pinned together. Because these are prefabricated off site in Derbyshire, it can be built surprisingly quickly: five metres (that is, two sections) a day is a good average, although here too things can be slowed down by bad weather. (You really don’t want to be swinging huge sections of concrete off a crane in high winds.)²

Again, though, things are more complicated than that makes it sound. Out of the walls stick orange reflectors, devices installed to monitor the structure as it settles. Some sections also have to be installed beneath bridges, which can’t be removed even temporarily because they carry utility pipes. To do this, one subcontractor has had to invent a special frame that allows them to install those vast chunks of concrete from below without a crane. Extensive wire netting holds the earth back on either side so rocks don’t bounce down on anyone while they’re working. Not for the first time I am struck by quite how much thought has had to go into all this.

This tunnel, incidentally, has prompted protests from a local action group, worried about ground water and construction traffic (deciding which is their bigger concern I leave to you). That led Buckingham Council to refuse permission for an underground chamber and ditch, against the advice of their own planning officers, on the grounds that building the access road would be “harmful”. Sure. A solution was found – more from Martin Robbins here – but only after HS2 Ltd argued that the decisions would mean delays and a bill for taxpayers running into “tens of millions of pounds”.

Multiply that by such complaints all along the line and you might start to understand how this thing got so expensive.

In his fascinating book about the science of knowledge, The Infinite Alphabet, Cesar Hidalgo notes that organisations, like individuals, “forget” things, and at a surprisingly rapid rate: around 3-6% a month, which adds up to around 50% in a year. One of the best arguments for continuing to do certain things, even if they are not always immediately profitable, is to prevent the decay of that knowledge.

I mention this because of something one of the men building the green tunnel told me, shortly before I got back on the train to London: that it had taken them a while to realise that the waterproofing process would be significantly quicker if they weren’t doing it while impeded by the rain. That’s why, in the picture above, it has a sort of temporary tent covering part of it.

Projects on this scale are, as you will have gathered, complicated. There are a thousand aspects to them invisible to those who merely use the resulting infrastructure. Equally, though, there will be a thousand tricks to do things faster or cheaper that you only learn by doing them.

Whatever you think of HS2, one of the best arguments for continuing to build something – for having a long-term infrastructure plan, so that contractors and managers can move from project to project, taking their expertise with them – is to build on that knowledge and make sure we’re not starting from scratch every time.

There’s a tendency, in other words, to look at the fact these things are expensive and complicated, and take from that we should do them less. That, I suspect, is precisely wrong.

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1. Complicating things further, near Calvert, Bucks, the new HS2 line will cross the other big current rail project, the East West Railway. EKFB is building a junction, as it’s easier to incorporate into the design than to add it in later; but the result is a stretch where the scheme suddenly doubles in width.

2. I’m over simplifying – there’ll also be a couple of emergency doors; access platforms along the middle and cabling at the edge needs installing too – but that’s the gist.