The Hampstead Junction Railway (HJR) "was intended principally to enable local passenger traffic on the North London railway to extend west to Kew and Richmond without the need to pass through Camden station and Primrose Hill Tunnel, where enormous traffic on the London and North Western's main line presented a serious obstacle to the running of local passenger trains at frequent intervals".

Congestion near Camden Town led to the promotion of the HJ.R which opened a northerly bypass through Gospel Oak and the central part of Hampstead to rejoin the main line at Willesden. Stations were opened in 1860 at Hampstead Heath and Finchley Road (from 1880 Finchley Road & Frognal). The line was tunnelled between Hampstead Heath and Finchley Road, burrowing under the hill of Hampstead, the highest point in the London area north of the Thames.

Gradients on the line rise from each end to a high point in Hampstead Tunnel. The steep rise in and out of Hampstead by road had caused a previous transport improvement - the Finchley Road which opened in the 1820s - effectively the Hampstead by-pass if its day.

Near to the westernmost tunnel entrance, the Kelebourne brook (downstream called the Westbourne), once flowed over the fields marked by the exact point of the map marker. The river is now buried under the suburban streets as a sewer.