Indian Railways, which consumes about 2.5 million tonnes of diesel every year, till last year bought the fuel from state-owned fuel retailers.
RIL won a tender to supply diesel to Railway locomotives this fiscal and "supplies have started," a company source said.
The government had in October last year freed diesel price from its control by ending subsidies. This led to rates at which state retailers sold the fuel coming to par with market price.
Since prices were deregulated, RIL was able to compete with PSUs and offered an attractive discount, which helped it win the Railway tender, the source said.
The company, he said, was currently selling "small quantity" of diesel to the railways.