Here is the award citation, here is one excerpt from it:
Donaldson’s paper “Railroads of the Raj: Estimating the Impact of Transportation Infrastructure?” (American Economic Review, forthcoming) investigates the economic benefits from building transportation infrastructure studying the case of railways in 19th century India. This paper is widely viewed as both a methodological breakthrough and substantively important paper in the field. Donaldson assembled a new and rich data set from archival sources about the expansion of railroads in India through the 19th and early twentieth century and the volume of inter-regional trade in the same period. He then uses the data to look the effect of access to railroads on real agricultural incomes. To check that this effect does not come from building railroads where the growth was predicted to be, he uses the fact that a number of proposed lines did not get built or did not get built when they were proposed to be built. Assuming that the proposal was based on what the contemporary experts thought were the areas of greatest demand for transportation, these un-built railroads should also have an effect if they were any good at predicting growth. He finds no such effect.
The second part of the paper builds a quantitative model where the effect of trade on real agricultural GDP is fully captured by one sufficient statistic: the share of expenditure that each Indian district allocates to goods produced in the district. When that share is low, it indicates that the relative price of imports in the district is low, and in turn, that the welfare gains from trade are large. Controlling for shocks to technology (mainly rainfall in this case), he finds that observed changes in real GDP following access to the railroad move almost one for one with the sufficient statistic predicted by the model, thereby making the case that the benefits of the railways is indeed the result of increased trade.
There is much more of interest at the link. Here are copies of the papers, overall I am delighted to see a Clark Award that so prominently features economic history, not to mention India and trade. Donaldson is at Stanford, here is his home page. An excellent pick, but this one was a surprise to me.