On this page of the Hitman 3 guide you will find a list of all the best disguises you can obtain while completing the Untouchable mission on the Carpathian Mountains (Romania) map. The suggested disguises will allow you to explore the train with ease, making it easier to approach your destination without arousing suspicion.
The black outfit of an elite guard will enable you to move fairly freely between the soldiers guarding the compartment wagons. You will encounter more suspicious soldiers in some compartments, but they can easily be evaded.
You can get the Elite Guard outfit without engaging in any combat, in the compartment with the sleeping guard, which is located just behind the bar compartment. You can enter it from the outside, through a window. The disguise lies on the chair on the right side. Its acquisition is linked to the Providence Guard challenge.
The disguise of a commando squad leader helps you move freely around a large area of the rushing train. The opportunity to obtain this disguise will appear approximately halfway to Arthur Edwards.
Hide in the location shown in the image above. Wait for one of the guards to start walking to the other end of the train car. Now, attack the man standing by the crate from behind and overpower him. Change into his clothes (I'm The Conductor Now challenge), and hide his body in the chest on the left.
When you exit the carriage, all the elite guards, who would normally pay attention to you, will stop being suspicious, allowing you to walk freely between them.