A、Bad guys in old films. B、A person’s life project. C、Religion or politics. D、People’s goal or purpose. C

Train and railway expressions worked their way into American English over a century ago. Many of those terms are still being used today.
    (20)Railroads changed the look of the United States. They also gave many Americans the chance to move around the country.
   Sometimes railroad tracks would not join up in a town but divide into two. If you say someone is from the wrong side of the tracks, it means they are from the bad side of town. To go off the track means to move away from one’s intended purpose or goal.
   For example, you could say your career went way off track if you wanted to be a lawyer but became an actor instead. But if you go to law school and complete your studies, your career is back on track. Let’s say you finished law school very quickly because you took double the course load. You then could say you fast-tracked your way to becoming a lawyer. In the world of business, it is common to say a project has been fast-tracked or is on a fast track.
   To stay on track means to pay attention to your goal or purpose. But getting sidetracked is just the opposite. A train that is sidetracked gets sent to a different station or down a different line.
   Talking about train tracks, a rail is a single piece of steel that is part of the larger railway. Some rails are dangerous to touch. The third rail is also an issue which can fuel a heated debate. (21)At a party, religion or politics could be the third rail topics. Talking about them could kill your social life! When something goes off the rails, it is always a bad thing. To go off the rails means a person’s project or life is a wreck, or—pardon the expression—a train wreck. When a train reaches its final stop, we say it has reached the end of the line. (22)If you reach the end of the line you have come to the end of something. Often in old films, this is what the bad guy says just before he kills someone.
20. How did Americans benefit from the railway?
21. What could be the third rail topics at a party?
22. What does it mean when we say a man reaches the end of the line?