Data can be sold to big corporation and advertising companies. This data can be used to target specific users to do target advertising. Documents of users can be used to create forged documents. Source
Yesterday (4 May 2016), the Cyber Cell of Mumbai police informed the Railway authorities that a large volume of data from its website was stolen.
So overall it is a yes or no that the site may be hacked or not hacked. The result will come after investigation.
Here is what IRCTC has to say in their press release.
As of my opinion if the website is hacked there must be a chance that this is done by some internal person. What you say about this? Do comment your opinion below!