How Does a Train Climb a Mountain? Several Answers, All Expensive

A railway works because steel on steel has very low rolling resistance, which is why a locomotive can move enormous loads on the level. The same low friction limits how hard a wheel can push before it slips, and that sets a maximum gradient for an ordinary adhesion railway at a few per cent, with main lines generally kept well below that and with anything above around four per cent being exceptional and requiring special operation. Mountains demand gradients many times greater. A line therefore either lengthens the route enormously to reduce the gradient, tunnels through to avoid the climb, or abandons adhesion and uses a different means of getting up.

Each approach trades cost, speed and capacity differently:

• •Spirals and switchbacks, which lengthen the route within a valley to keep the gradient workable
• •Rack and pinion, with a toothed rail engaged by a gear beneath the locomotive
• •Cable haulage on a funicular, with two cars counterbalancing each other on a cable
• •Aerial cableways, which abandon the ground entirely
• •Base tunnels, which avoid the mountain rather than climbing it
• •Combinations, with rack sections on an otherwise adhesion line

The rack is the classic mountain solution and its details matter. A toothed rail is laid between the running rails and a driven gear on the locomotive engages it, which transmits force through the teeth rather than through friction and allows gradients of twenty five per cent and in one case forty eight. Several designs of tooth exist, differing in how the load is shared and in how smoothly the gear enters, with the simplest using a flat rack with teeth cut into it and others using a ladder arrangement or double racks for higher loads. The same system brakes the descent, which is the more important function, since a vehicle descending a steep gradient on friction brakes alone would overheat and lose control.

The cable solution deserves a description because its economy is elegant. Two cars are connected by a cable running over a pulley at the top, so one descends as the other ascends and each counterbalances the other, which means the motor supplies only the difference in weight plus friction rather than lifting a full car. Water ballast was used in early examples, with the upper car filling a tank and the lower emptying it, so the whole system ran on gravity and on a supply of water at the summit, and several such lines operated for decades with no external power at all. A single track with a passing loop at the midpoint serves both cars, with the wheels arranged so each car takes its own side automatically without any points that move.

Almost none of these railways were built for the reasons that justify ordinary lines. Several were built for tourism from the start, in the nineteenth century when mountain scenery became fashionable and when reaching a summit by train was itself the attraction, and a number of the most famous carry no other traffic. Others served mines, quarries and forestry and were abandoned when those closed. Some serve communities with no road access, particularly in Switzerland, where the railway is genuine transport infrastructure and is treated as such. Several were built to reach observatories and transmitters. The economics were frequently poor from the beginning, and a substantial proportion of the lines built have closed, with a number preserved and reopened as heritage operations.

Steel on steel slips above a few per cent gradient, which rules out mountains for an ordinary railway. Lengthening the route with spirals, engaging a toothed rail, hauling on a cable and tunnelling beneath are the available answers. The rack matters most for braking the descent, since friction brakes alone would overheat, and most such lines were built for tourism rather than transport.