It's a bit dated and not too detailed, but check Another Great Game. The Indian subcontinent indeed has broad gauge (the gauge change is in the Iranian city closest to Pakistan), but political barriers are a bigger obstacle. Shipping will always be cheaper, rail can win on delivering faster.

Nevertheless, the governments want to make it happer, and this year:

Railway Gazette: A long way to go

...following the announcement in June that the Bam - Zahedan line in southeast Iran had been completed, the first international freight train ran over this route in August.

On August 14, Pakistan's Prime Minister Syed Yusuf Raza Gilani marked the country's national day by flagging off an inaugural container train from Islamabad to Tehran and Istanbul via Zahedan. Expected to take 15 days, the train was operated under the auspices of the regional Economic Co-operation Organisation.

Originally established in 1985 by Pakistan, Iran and Turkey, ECO later expanded to encompass seven Central Asian states. It is actively promoting the operation of other long-distance container trains across the region, including Almaty - Bandar Abbas and Istanbul - Urumqi.

Although the initial train from Islamabad was a demonstration run, it carried 20 containers with 750 tonnes of freight. PR General Manager Saeed Akhtar is confident that there would be sufficient traffic for a regular service linking Lahore or Faisalabad to Istanbul.

_{*Lunatic*, n.
One whose delusions are out of fashion.}