The French TGV is a inter-city high speed railway that link the cities in French and neighbour countries. In 1981, the LGV Sud-Est, which links Paris’ and Lyon’s suburbs, had opened. Since then, it becomes the busiest high speed rail line in France, and it soon reached the highest capacity. Eventually, the engineer had to design a new double-decker train, which later named the TGV Duplex, and it helps to increase the capacity to more than 45%. The TGV Duplex had used many advance technology during that time, like using aluminium as the train bodies. It also broke the world fastest train record with 574.8 km/h in 2007.
Why do you think the engineer decide to use a double decker train? Why don’t they extent the train to increase the capacity or use other solutions?
Jasper's Blog on Trains 