Conceived as a servant of local paper mills, the Esk Valley Railway received Parliamentary approval in July 1863 but work on it did not get underway until 5th September 1864. From a junction with the Peebles Railway, its single track extended for three miles to a terminus at Polton but getting there demanded considerable engineering including a tunnel and striking curved viaduct. These were substantially complete by the summer of 1865. Contracted to build them was Mr Alexander Wilson of Edinburgh who would receive £20,994 for his troubles.
The tunnel - engineered by the prolific Thomas Bouch - is 430 yards in length and follows an S-shaped alignment. Both curves are about 18 chains in radius, although the one at the east end only extends into the tunnel a short distance. The west curve starts 130 yards from the exit.
Approached through a cutting from the former site of Broomieknowe Station, the east portal is a simple linear structure, stone-built and lacking any buttresses. As a consequence, ground movement and tree growth above have prompted a deep circumferential crack to develop behind the voussoirs.
The lining is brick, considerable sections of which have been patch repaired due to spalling. At the crown, several lengths exhibit open joints; there is also a long developing fracture towards the western end. At a number of locations, deformation of the sidewalls has resulted in ledges forming at springing level. These deformations are either angular in form or bulge-like.
About 100 yards in, there is a considerable 'kink' where the tunnel shears to the north by almost two feet over two lengths. This is presumably the function of a setting-out error during construction. A number of open catchpits can be found at the toe of the north sidewall whilst telegraph wire supports and insulators are hosted on the opposite haunch. Although the tunnel is mostly dry, calcite and stalactite formation 350 yards from the east end suggests it is susceptible to localised water ingress.
The west portal is also affected by root displacement but is in generally better condition than the east portal. It benefits from wing walls running parallel with the former trackbed, helping it to better resist ground movement. The approach cutting is short but accommodated the end of Lasswade Station's former platform. A short section of this is still extant within the tunnel.
The Esk Valley Railway opened on 16th April 1867, leased to the North British Railway for an annual rent of £1,350. The works had actually been completed seven months previously but delays were encountered with improvements to the Peebles Railway - from which its traffic flowed - and the installation of signalling equipment demanded by the Board of Trade.
The line carried its last passengers in September 1951 but goods trains continued to serve the mills until 18th May 1964. Since then the tunnel, structure EVP/3, has continued to be maintained by the Highways Agency Historical Railways Estate, formerly BRB (Residuary).