Back when railways were being massively build, in the 1900-1910 decade for example, most new lines were made with narrow gauge, instead of the standard gauge which is 1435mm for most of the world.
For instance, in Switzerland, most new train lines built between 1890 and 1914 used a gauge of 1m. Urban tramways used 1m gauge too.
The reason for this is that this was much less expensive. I wonder however what factors lead to the construction of such smaller railways to be less expensive. Understandably the earthworks are lighter, but not so much as the width of the vehicles dictates this and not the width of the tracks.
Quotes as for where I learned that narrow gauge was supposedly cheaper : From Wikipedia page on narrow gauge railways.
Since narrow-gauge railways are usually built with smaller radius curves, smaller structure gauges and lighter rails, they can be less-costly to build, equip and operate than standard- or broad-gauge railways (particularly in mountainous or difficult terrain).
La voie normale permet de mettre en service des trains directs depuis Bâle jusqu'à Saignelégier pour les jours de Marché-concours. Mais la construction a coûté cher. La compagnie est rapidement en difficultés financières et n'a plus d'autres choix que demander de l'aide à sa rivale. Les deux compagnies font direction commune dès 1906.
(My translation (emphasis mine) ) The normal gauge allowed to have trains directly from Basel to Saignelégier on days of Market. But the construction costs were prohibitive. The company was quickly in financial difficulties and had no other choice than asking the rival company [translation note:The narrow gauge railwy Saignelégier-La Chaux de Fonds] for help. Both companies had common leadership since 1906
If a new line were to be built today, now that massive industrial construction machinery is available, would it still be significantly less expensive to make it narrow gauge ?