If your travel agency is in charge of providing the trains, then they will ask you for the necessary documentation, which is usually only a photocopy or image of your passport. This document must be current and belong to the person who will use the service, since the train station will validate your data by the staff of the train companies, if they do not match, then you may have problems boarding the train to Machu Picchu. The train ticket is usually round trip, it is only back when you do the four-day Inca Trail, since you will arrive on foot. Do not forget to have your passport and identification document at hand, because it is very likely that you will be asked to prove your identity on all tours in Cusco and Machu Picchu.