To unlock the Brakeman trophy in Metro Exodus, you must detach the two train cars found towards the end of the game's first level.
During the train section, you will be edging along the outside of a train until you come to a window with a guard standing in front of it. Take down the guard and enter the train. There is a collectable to the left that you might want to grab first before progressing any further. Carry on to the right and step across to the next carriage but don't open the door.
Instead of opening the door by holding the Square button, look slightly to the left-hand side at the red handle. Press the Square button to detach the first train car. Once done, you'll be notified by Miller over the radio that you are doing a good job.
Continue forward through the train, squeezing through the tight gap. Once you pass through the plastic curtain will see a similar door and level to the one above. Use the lever on the left-hand side again to detach car two.
Once you have detached both train cars, you will unlock the Brakeman trophy.