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Hillclimbing is a problem faced by railway systems when a load must be carried up an incline. While railways have a great ability to haul very heavy loads, this is only possible when the tracks are fairly level. As soon as thegradients increase, the tonnage that can be hauled is greatly diminished.
Early tramways and railways were laid out with very gentle grades because locomotive and horsehaulage were so low in tractive effort. The only exception would be with a line that was downhill all the way for loaded traffic. Brakes were very primitive at this early stage.
Where a railway has to cross a range of mountains, it is important to lower thesummit as much as possible, as this reduces the steepness of the gradients on either side. This can be done with a summittunnel or a deepsummit cutting.
A summit tunnel can lower the summit even more, and steeper hills result in shorter tunnels. Also, tunnels cost the same no matter how much overburden there is, while cuttings tend to increase in cost with the square of the overburden.
Care had to be taken with summit tunnels in the early days of steam with designs that suffered from problems with smoke and slippery rail.
Theruling gradient of a section of railway line between two major stations is the gradient of the steepest stretch. The ruling gradient governs the tonnage of the load that the locomotive can haul reliably.
Some of the techniques that can be used to overcome steep hills are: