Shankar Sharma
I live is Delhi so I think I am qualified to answer this Question.
The answer from @kirti agarwal is correct that Chandni Chowk metro station is the nearest metro station to Old Delhi Railway Station but the DISTANCE is not 2 km.
The Old Delhi railway station is just few minutes away from the metro station. All you need to do is just exit from gate number 3 and you will reach the station is just couple of minutes.
Source :