Walkers crossing the open grass of Primrose Hill are rarely aware that, a short distance to the north-west, under the ridge running towards South Hampstead, trains have been passing through the ground since the 1830s. The Primrose Hill Tunnel is unglamorous and almost invisible, but Historic England records it as the first railway tunnel in London, and the first anywhere in the country to have had to settle the competing claims of a railway company and a landowner in an urban setting.
Why Eton College Insisted on a Tunnel
The ground the London and Birmingham Railway had to cross north of Camden was the Chalcots Estate, land belonging to Eton College. The college's interest lay in the building value of those fields, and the Act of Parliament of 1833 that authorised the line left it in a position to impose terms. Two of those terms shaped everything that followed. The railway was to be tunnelled rather than driven through in open cutting, and the eastern mouth was to be, in the wording preserved in the listing record, "made good and finished with a substantial and ornamental facing of brickwork or masonry to the satisfaction of the Provost and College".
That is an unusually specific piece of nineteenth-century development control, and it is the reason the eastern portal at Chalk Farm looks the way it does. It is equally the reason the western end, where no such condition applied, is comparatively plain.
Fighting the Blue London Clay
The contract was let on 21 April 1834 to the contractors Jackson and Sheddon, and work began that year with Robert Stephenson as chief engineer. The bore was driven from four vertical shafts, each eight feet in diameter, with cut-and-cover used for roughly twenty-one yards at either end. Accounts give the finished length as about 1,164 yards, a little over a kilometre; some sources round it down to 1,100 yards.
The London clay proved far harder to work than expected. It was impermeable but extremely tough, and contemporary accounts describe crosscut saws and hatchets being used on it rather than conventional excavating tools. Worse, the moist clay swelled once exposed to the air. The pressure squeezed mortar out of the brick joints and crushed the bricks it bore against. The response was to abandon hollow London stock brick in favour of very hard paver bricks, to replace lime mortar with quick-setting Roman cement that would harden before the ground could deform it, and to build an arch twenty-seven inches thick. The working face was not permitted to advance more than about six and a half feet ahead of the completed lining.
Caution on that scale was expensive. The six-mile section of line that included the tunnel was originally let for £120,000 and is recorded as having cost more than £280,000 by the end; the ground conditions are said to have bankrupted the appointed contractor. Excavation was complete in January 1837, and the tunnel is recorded as opening in 1838.
The Ornamental Portal at Chalk Farm
The eastern portal was designed by William Budden, an assistant engineer to Stephenson, in an Italianate manner, and cost in the region of £7,000. It is built of Portland stone and London stock brick, with rusticated voussoirs and carved lion masks. Historic England regards it as the first tunnel portal in the country to be treated architecturally — designed as a piece of civic architecture rather than merely finished off. The intention was to make the railway acceptable to the smart residential development Eton College hoped to raise on the Chalcots fields above it.
The Second Bore and Grade II* Protection
Traffic grew, and in 1879 the London and North Western Railway added a second bore on the southern side, engineered by William Baker, the company's engineer-in-chief. Its portal faithfully repeats the design of the original but stands taller, because of how the ground had to be retained. Each bore carries double track, and the route is now part of the electrified West Coast Main Line.
Both sets of portals were listed on 14 May 1974: the eastern portals at Grade II*, the western at Grade II. The listing rests not only on their appearance but on their place in the history of how railways were fitted into cities.
What the Tunnel Left Behind in NW1
The tunnel is only the most hidden part of a much larger railway presence in this corner of Camden. The approach from Euston, the railway lands at Camden and Chalk Farm, and the bridges and retaining walls around them all belong to the same push north-west. Primrose Hill had its own station on the line, long closed to passengers and since put to other use. The Chalcots Estate was in due course built up much as Eton College intended, and the streets of Primrose Hill sit today above a piece of engineering that had to be negotiated clause by clause before a single brick was laid.