The first enemies are all found in railcars, shooting at Artyom and Khan. The player must quickly kill all 4 soldiers in railcars on the track to the right before Khan blows up a railcar infront with his Ashot. After that the player must quickly kill the other 4 soldiers on the right and the train machinegunner. Right after that there are several soldiers shooting from the train. They too must be killed quickly. Finally you jump on the train - the rest are easy as they will all be in your way. Once the last one (the one who breaks down the door with a Saiga) is dead, the achievement/trophy will unlock. Below is a video guide if you are having trouble.