Exercise 1: sample answer

In Chapter 2 the early lines were built to narrow gauge because it was cheaper in mountainous country. That choice limited speed and capacity, so by the 1950s the main corridor was overloaded, and building a new standard-gauge line on its own track (Chapter 4) was the way to escape the limit. A cost decision in the 1870s helped create the case for the Shinkansen (check the reasoning).