Agent! how to buy a train ticket by euro pass

You get the Europass already, you don't need to buy train ticket anymore.
If you want to travel on TGV or other night trains, you just go to the box office of the train stations. Then tell him the time and date of your travel, and tell him you need to "reserve the seat". Then he will help you to reserve the seat. Take the ticket he give you, go to the platform to find the train, please stamp your ticket on the orange machine on the platform and write down travel date on the Europass. then get on the train.
If you travel in normal train, just get on the train directly..
P.S. It's "Urgent"... not "agent"...

You can get on the train directly and need not buy any ticket.