Although not a big engineering achievement, my local line to Hastings provided a bit of a challenge.
When the Tonbridge-Hastings line was built in the mid 1800s (opened in stages between 1845 & 1852) the contractors tried to save money by cutting corners when building the tunnels on the route. These should have been lined with 4 rows of bricks, but we're only lined with 2 rows. When some of the tunnels later started to collapse they were then reinforced with a further 2 rows of bricks, bringing them up to the correct specification, but now narrower than they should be. (Mountfield tunnel was one of these affected in this way).
This restricted the width of trains that were able to pass through the tunnels & in an era when trains were getting bigger, special designs were needed - the Southern Railway designating this width restriction as 'Restriction 0'. This problem persisted through Southern Railway ownership & into the British Railways era, up until 1986 when the double track through the affected tunnels was singled, meaning and end to restriction 0.
Some of the locomotives designed specifically for this route were the SR V (Schools) class, BR Class 33/2 & the BR Class 201 (Thumper) Diesel Multiple Units.
For further reference, Hastings Diesels Ltd (
www.hastingsdiesels.co.uk) would be a good place to start- they run a preserved Thumper and their website has some history sections too.
Tony