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 a fortnight 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. But the mad idea of terminating trains at Old Oak Common, rather than Euston, has thankfully been dropped – the tunnel boring machines will begin their journey east early next 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.