An ordinary train climbs very badly. A steel wheel on a steel rail has very little grip, and that imposes a gradient limit that in practice is around two or three per cent: twenty or thirty metres of climb for every kilometre travelled.
To beat that limit there is the rack: a third toothed rail that a pinion engages. It works, but it is expensive, slow and severely limits what can be hauled. On the C-14 line there is not a single metre of rack, and yet the line climbs more than three thousand metres.
01 The problem: climbing three thousand metres without a rack
Stating the problem is arithmetic. Salta station is at 1,187 metres; the line's highest point, at 4,220. That is 3,033 metres of climb.
At a maximum gradient of two and a half per cent, those 3,033 metres require some 120 kilometres of continuously climbing track. And the terrain does not offer 120 kilometres of valley in a straight line: it offers a narrow gorge that runs out long before that.
So the real problem is not gradient, it is length. You have to fit far more track than the valley holds. The C-14's engineers solved it with two geometric devices that appear on other mountains around the world, but rarely together.
The problem was not the gradient: it was that there was not enough room in the valley for the track.
02 Solution one: the zigzag
A zigzag is, literally, a dead end. The train climbs on one track up to a set of points, runs into a stub track that ends against the hillside, stops, and then the driver reverses direction.
Leaving in reverse, it takes the other arm of the points, which starts at a different gradient and climbs in the opposite direction. The train has gained height without needing horizontal space: it has folded the track back on itself.
The line has two, at El Alisal and Chorrillos. The first is at 1,806 metres and the second at 2,111: between them there are barely more than thirteen kilometres of track and three hundred metres of climb.
The price of a zigzag is time: each one means stopping, changing cab and starting again. On a freight line that is paid for in hours. It was cheaper than a rack.
03 Solution two: the loop
The loop, or helical spiral, solves the same problem another way. Instead of folding the track back on itself, it coils it: the alignment describes a complete spiral and passes over or under itself again, one level higher.
The advantage over the zigzag is that the train does not stop: it goes in below and comes out above without reversing. The disadvantage is that it needs a hillside with the space and the rock to trace the complete curve.
The C-14 has two, on the stretch between Tacuara and Diego de Almagro. Added to the zigzags, the tunnels and the viaducts, they make up a fairly complete repertoire of everything twentieth-century engineering knew about gaining height.
04 Two hundred and twenty-three metres of iron at four thousand two hundred and twenty
The La Polvorilla viaduct is the line's best-known piece and deserves the full figures. It is 223.50 metres long, weighs 1,590 tonnes and rests on six piers, the central one 63 metres high.
The deck is made up of six 14-metre spans and seven of 20. It was built between 1930 and 1932 by the Italian Monfalcone works and completed on 7 November 1932.
One detail sums up the whole work: the western abutment is four and a half metres higher than the eastern support. The viaduct is not level. It is on a gradient, because the train crosses it climbing.
05 Twenty-seven years of work
Construction started in 1921, with the American engineer Richard Maury heading the project from 31 March of that year. A station on the line bears his name, at 2,358 metres.
Maury did not see it through. He was removed from his post in 1930 and did not take part in the inauguration of the La Polvorilla viaduct two years later. The Chilean section opened in 1947 and the complete line on 20 February 1948: twenty-seven years from the first day of work.
The result is 941 kilometres of metre-gauge track, 571 in Argentina and 330 in Chile, connecting Salta with Antofagasta. It is the fifth-highest railway in the world and the third in South America.
06 What to look at if you go up
First, the viaduct's own gradient. It is visible to the naked eye by comparing the railing with the horizon: the deck falls towards the east. It is not an assembly error, it is the line's profile.
Second, if you pass El Alisal or Chorrillos, look for the track that leads nowhere. The zigzags leave a stub of track ending against the hillside, and that dead end is the piece that makes all the rest possible.
And third, the whole. Current tourist services cover the high section, some 21 kilometres, at 35 kilometres an hour. That is a ridiculous speed for a train and a perfectly reasonable one for what is being travelled.
