In the 1840s the Southampton and Dorchester Railway constructed a railway tunnel through this same high ground, cutting through the route of the canal tunnel at an acute angle towards the western portal. Early reports by the railway's engineer suggested it might be possible to use the canal tunnel in some way for the construction of the railway but by the time the Bill for construction was placed before parliament this idea had been rejected and a new alignment with a shorter tunnel was proposed. A contemporary scale cross sectional drawing, made in June 1847, shows that close to the point at which the two tunnels met the base of the canal tunnel was only slightly lower than that of the railway tunnel and that the level of the water it contained was at a similar level to the base of the new tunnel. The specific alignments and levels of the two tunnels meant that water was able to accumulate beneath the level of the newer, larger, structure and saturate the surrounding gravels and clays. Just prior to the railway tunnel's planned opening date, saturation of the ground around the area of old the canal tunnel was reported to be responsible for a collapse of a recently constructed 100 yard section at The London Road on 23 April 1847. After repairs were completed and a formal inspection had taken place, a further 60 yard section began to sink and opening was delayed for two months whilst this was also dealt with. Captain Coddington, the Government Surveyor of Railways visited the site to report on the situation and recorded what had become of the canal tunnel: