Why is this the answer?

The answer is train because this exciting riddle is describing a huge, powerful machine that moves people and cargo across the land, but it uses clever word pictures to make you think about something mysterious. Let's act like detectives and look at all the clues one by one to solve this puzzle. First, the riddle calls it a "strange creature" that is "Long, hard, and straight." Think about a train. It is made of lots of cars linked together, making it very long and straight, and its metal body is hard. It is a giant sight on the tracks, like a strong, amazing creature made of steel. Next, it mentions the creature "thrusting into a round, dark opening." Where does a long train go that is dark and round? A tunnel! The tunnel entrance is the dark opening that the train engine pushes itself into to get to the other side of a mountain or under a city. The riddle then says it is "Preparing to discharge its load of lives." A train is filled with many people, and people are living beings, or "lives." When the train pulls into a station, it is ready for everyone to get off and go home or start their adventure. It is "discharging" its passengers when the doors open. Finally, we have the sounds: "Puffing and squealing noises accompanied it, then a final screech as it slowed and stopped." A train engine makes a loud, steady "puffing" noise as it chugs along, especially the older ones. When the driver applies the brakes to stop at the station, the metal wheels rub against the track and the brakes, making a very loud, sharp "squealing" noise. That final, big sound is the train coming to a gentle halt. This riddle is a creative story about a train arriving at the end of its journey.