Note: Getting this memento will alert everyone in the station, blowing your cover. The train will also attempt to leave prematurely, forcing you to fight your way through alerted enemies before the timer expires. If you want to maintain stealth (recommended) for an easier mission, get the memento then reload the checkpoint/ restart the mission.
Sneak through the first house and once you’re outside, carefully head to the left where there are three enemy soldiers standing guard. Kill all of them (base will be alerted) then get the poster from the bucket beside them.
This is found in the second house you’ll have to enter, after passing through the field filled with german soldiers and supplies. Upon entering the house, search the left corner to find this memento on top of a piano.
After taking down the train and joining up with Rosseau, you’ll have to fight through the surviving german soldiers along the way. Continue until you reach a large enemy force past the broken train car that you have crawl under to get past. Take out most of them to force the remaining ones to retreat. Before chasing after them up the hill, check out the wooden crates they used as cover to find the blueprint on top of a wooden crate.