i bought a train ticket and i want to return it. i want to know how much percent do they take

If you do it at the railway station, it depends on the time you return it. If you do it more than 48 hours in advance, you can get 95% back. If you do it between 24 to 48 hours, you can get 90% back and if less than 24 hours, it would be 80%.

Forget one thing, if you able to return it more than 15 days in advance, you can get full payment back. However, if you make the cancellation via your booking agent, the cancellation fee depends on their policy instead. You need to contact them directly.