Most of the long distance trains in India have bookings opened 90 days before the train ride. There are some that have a shorter period available.
Which exact Indian trains have reservations open only 30 day prior to departure?
According to the Indian Railways site
You can get a partial list (though not official) of Inter city trains from Wiki
There may not be a hard rule which trains fall into the 90 or 30 day category. If you are interested in a particular train, you can try to book it online 60 days in the future and see whether you can book it. You can try cleartrip or the official Indian railway site
Interestingly enough cleartrip allows me to book 'Delhi to Kalka' in April 2012 (88 days from now), so the 30 days mentioned above may no longer be correct.
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