The railways came about in their modern form with the development of the steam locomotive, a new and untried piece of technology. At the same time many of the civil engineering techniques required to build a railway were well-developed. This combination of the new and the old had a considerable impact on the design of the early lines.

The first steam locomotives were neither very powerful nor reliable, and operations could be difficult if the railways did not accommodate their weaknesses. In general, the early railways were built to easy gradients that the locomotives could manage. This could require major earthworks, as well as tunnels and viaducts. The following table shows that, with the exception of the London & Croydon Railway, the main lines built south from London before 1850 all had gradients no worse than 1 in 200. As more powerful locomotives were developed, it was possible to build railways at less expense, with steeper gradients. Railways that would not have been economical at all if built to easy gradients could now be built.

The London & Croydon Railway was in cuttings most of the way from New Cross to Forest Hill, but there was little that could be done to overcome the difference in height of about 120 feet over a distance of only 2½ miles. The small engines used on the line managed somehow, though trains were reported to weigh less than 20 tons gross. Average speed over the line as a whole was 21 mph, which suggests that progress up the bank was considerably slower. It is, perhaps, not surprising that the London & Croydon was one of the few railways to try out atmospheric propulsion.

Although South East England is quite low lying, some areas are very hilly. This could require considerable earthworks if a moderately-graded railway was to be constructed. The London & Brighton Railway had to build its line through the North and South Downs and the Weald, going directly across the grain of the land. There had been proposals to build a railway to Brighton through the Shoreham gap, but that would have been a very indirect route. The line engineered by John Urpeth Rastrick was not only easily graded, but it followed a direct route and had no sharp curves.

The cutting that takes the Brighton line through the South Downs south of Clayton Tunnel is extraordinarily deep, but it was dug entirely by hand. Apart from vegetation growth it is little-changed from when it was first excavated. This sets it apart from many other early cuttings, such as Tring (London & Birmingham), Sonning (Great Western) and Weybridge (London & Southampton), which have been widened. The London, Brighton & South Coast Railway secured powers and purchased land to quadruple the southern end of the Brighton early in the twentieth century, but work had not started by the outbreak of the First World War and the scheme lapsed. Photographed 29th August 2011 photograph by Gregory Beecroft

photograph by Gregory Beecroft

The Brighton line is noteworthy for having some of the most extensive earthworks in near original condition on an early main line. The majority of the trunk railways from the 1840s have developed into the main lines of today. Most have been widened or modernised, but the cuttings that take the Brighton line through the Downs are little changed. When the railway was quadrupled south of Coulsdon, the additional tracks followed an entirely separate route to Earlswood, reaching a higher summit and with gradients of 1 in 100.

Bletchingley Road and School Hill bridges, Merstham. These bridges at Merstham, photographed on 26th August 2007, illustrate the different heights at which the London & Brighton Railway (via Redhill) and the Quarry Line cross the North Downs. Bletchingley Road bridge, in the foreground, carries the Quarry Line and is of sufficient height not to need any signed restriction. School Hill bridge, in the background, is a low arch carrying the line via Redhill. It gives just 3.3 metres clearance in the middle of the road. photograph by Gregory Beecroft

The London & Southampton Railway adopted a different approach, not attempting a direct route through the hills of North Hampshire. The railway followed a westerly route from London, before turning south at Basingstoke. This enabled a steady climb to a summit at Popham, whereas a route to the east would either have adopted a saw-tooth profile or involved exceptionally heavy earthworks. It was not until 1865 that the Mid-Hants line, via Alton, was completed with gradients as steep as 1 in 60. Another factor in deciding the route of the London & Southampton line was avoiding the estates of hostile landowners.

The South Eastern Railway' line to Dover was even more indirect than the London & Southampton Railway. The railway branched off the Brighton line at Redhill, avoiding, for the time being, the cost of building a line through the North Downs. This resulted in a route with little in the way of major works, save between Folkestone and Dover. By the time that the South Eastern was obliged to build its own direct railway to Tonbridge, steeper gradients could be contemplated. However, extensive tunnelling and deep cuttings were necessary to pass through the hills. Sevenoaks tunnel is the longest on the Southern Electric system and for a time it was the fourth longest in the country.

Electric traction made it possible for trains to operate over steeper gradients without banking or other special arrangements. The Wimbledon to Sutton line, completed in 1930 and worked by electric trains from the start, has gradients of 1 in 60, with a climb of 1 in 44 to the junction at Sutton. The gradient between City Thameslink and Blackfriars is 1 in 29, this being necessary to allow the railway to pass under Ludgate Hill and over Queen Victoria Street.