No Thanks. Rail links are easy means to rush troops to the border. Especially when the rail link crosses it.
Before anyone comes up with a "but the link works both ways, and we can rush our troops to the border using it, too" argument, let's face it: Nearly no part of our territory bordering China has terrain fit for 6' broad gauge lines. Not at Uttarakhand, not Ladakh, not the North East, either. At best the rail line on our side could go a few dozen kilometres into Indian territory, beyond which our terrain wouldn't permit the line to go further (unless we have a line leading to it that cuts through mountains over hundreds of bridges and tunnels. On China's side, on the other hand, Xizang (Tibet) already has a much more developed rail network that connects to China's trunk rail lines that connect its western frontier provinces to its eastern heartland. So China has a higher military advantage with this rail link, than India having any trade advantage with it.