Join me on a walk through the atmospheric Kelmarsh Tunnel, beginning at the northern portal on the northbound side of the long-disused Northampton to Market Harborough railway line. This hidden gem of railway heritage lies nestled in the Northamptonshire countryside, now part of the Brampton Valley Way, a 14-mile trail tracing the route of a once bustling railway.

The tunnel I explore here is the Kelmarsh Down Tunnel, the original of two bores completed in the 1850s by the London and North Western Railway (LNWR). It was part of a broader plan to improve regional connectivity, with the full line opening to passengers and goods traffic in February 1859. At just over 480 metres (around 1,575 feet) in length, the tunnel cuts through a ridge near Kelmarsh village, one of two tunnels on this line, the other being the slightly shorter Oxendon Tunnel further north.

A Costly Collapse:
What makes the Kelmarsh Tunnel particularly fascinating is its troubled construction. On Sunday, 11 April 1858, while still under construction, the tunnel suffered a major roof collapse. The incident occurred during excavation through a particularly difficult section of blue clay, a material known for becoming dangerously unstable when wet. The collapse caused significant delays and required extensive reinforcement work before the tunnel could safely open the following year. If you look closely at the tunnel walls today, you might spot patchwork brickwork repairs, a silent testament to the drama that unfolded over 165 years ago.

From One to Two Tunnels:
Originally built as a single track tunnel, the line became so busy that a second bore, known as the Kelmarsh Up Tunnel, was constructed in 1879 to accommodate double track operation. This parallel tunnel, which lies a short distance to the east, features a ventilation shaft visible from above ground, unlike the older Down Tunnel you see in this video. It’s believed the original bore lacked a ventilation shaft either due to construction constraints or because it was later filled in or removed. Railway workers once nicknamed these narrow, damp tunnels “the rat holes”.

Decline and Rebirth:
The Northampton to Market Harborough line remained in use for over a century. However, with declining passenger numbers and the impact of the Beeching cuts in the 1960s, its future was cast into doubt. Passenger services were withdrawn in January 1973, and the line closed completely in August 1981. Rather than fading into obscurity, though, the route found new purpose. In the 1990s, the disused trackbed was transformed into the Brampton Valley Way by Northamptonshire County Council in collaboration with Sustrans, creating a much loved walking and cycling trail through a landscape rich in nature, heritage, and quiet mystery.