Railways are back in fashion. Globally, the industry has been booming, thanks less to high oil prices than to a growing emphasis on the environmental benefits of trains over planes. The UK now has its first high-speed railway line (a few decades after everyone else), Barack Obama is promising similar links in the US, Japanese-built bullet trains are making a splash in Taiwan, and the French seem never to have lost their love of fast trains. Then, of course, there are the rather slower trains operated by the state-owned Indian Railways, the world's largest commercial employer, with 1.4 million staff.

But it is the rail system of a bygone India that has attracted my attention recently. Colonial India – which comprised present-day India, Pakistan and Bangladesh – had no railways in 1850 but more than 60,000km of track by 1930. What difference did that expansion make to the country's economy?

Dave Donaldson is a young economist who now knows more about the details of colonial railways than anyone alive. For his PhD research on the subject at the LSE, Donaldson had to build a massive database based on paper records of the railway building programme, gathered in painstaking detail by colonial officials.