[2] Primrose Hill Tunnel's environmental context has transformed substantially, now obscured by fences, buildings and vegetation, yet the structures remain almost unchanged from when first built, aside from the duplication from single to twin portals.

[2] Choosing a route to Birmingham Curzon Street out of Euston station was its chief engineer Robert Stephenson,[7] who needed to pass through South Hampstead, a rural area which was starting to be developed concurrently with the railway's arrival.

Although bringing the advantage of being impermeable and keeping the work free from water, it is a tough material and was difficult to remove with the existing technology of spades, pickaxes and blasting, instead requiring crosscut saws and hatchets and increasing the expense, but the pressure of the clay also caused it to expand upon excavation and exposure to the atmosphere, necessitating additional unusual measures.

Firstly, an extra-thick brick lining of 27 inches (690 mm) was installed as the contractors went along, to hold back the pressure, and very strong timbering was needed to support the arches until a brickwork section was complete to prevent collapses.

For a short period, Stephenson was apprehensive that stress could cause the brickwork to cave in altogether, as it made small fragments fly off the facing and coat his clothing while he inspected the work.

All four doctors, whom included William Lawrence, John Ayrton Paris and Thomas Watson, signed their names to confirm that even with a locomotive letting off steam, the tunnel remained unaffected, dry and at an agreeable temperature.

In fact, they were firm that the dangers of passing through well-constructed tunnels were no greater than those of travelling on an open railway, and the company used this to set the public's mind at rest.

[2] By 1852 the London and North Western Railway, which had amalgamated from the L&BR, was experiencing rising pressure on this section of line and at Euston, and began to advocate for a second bore at Primrose Hill Tunnel to double the number of tracks.

[1] HE noted that the comparison of east and west emphasises the exceptional circumstances at the eastern end of the tunnel where the Eton College Estate demanded a grand architectural set-piece.

In its summary of importance, HE considers the eastern portal to be of more than special architectural interest for its proud, classical elevation which is indicative of the upmarket development the College hoped to undertake nearby; and that there are similarities in design with Brunel's portals to the Box Tunnel in the use of ashlar to imply strength, the classical features such as the treatment of the cornices and rusticated quoins, and the employment of quadrant arches to convey the sense of a grand entrance.