You can check the availability of seat in any Indian railway train of any date by checking here.
update- Now the best option to know the availability of seats in all train at any time period range at single web page is provided by- cleartrip train calender. Check here . Don’t forget to click fresh button ahead of train name in cleartrip train calender to get real time status of availble seats between the two stations.
The second option to check the seat availability is by using rediff website, check here.
The fact is that www.indianrail.gov.in provides seat availability information in “Train Between Important Stations”. Many people are unable to know that they need to use that link for checking seat availability. Most of the time, site is unable to deliver result, server remains down for most of the time.
Rediff search is good option for this perpose and dispalys result in more user friendly way with information of fare also on the same page. But Rediff result is also dependent on the availabilty of indianrail.gov.in server ON.