What purpose did the british have for railroads in india?

Faster movement of troops. 
Faster movement of goods. 
Cheaper transport of raw materials.

o better control the nation. India at the time of the raj was a disorganized nation with a very basic transport infrastructure. To get troops, merchandise and issue orders took too long on the very basic road system, so railways allowed them to centralize and strengthen their control of the nation. Commercially it also allowed them to exploit India as a market.

So why are you answering your own question?