This is the 7131st most frequent Japanese word.
レール
Rail.
In this sentence, 'レール' refers to the physical rails or tracks on which trains run.
列車がレールの上を滑らかに走っている。
The train is smoothly running on the rails.
Here, 'レール' metaphorically represents the structured path or norm usually followed in life.
人生のレールを外れるのも時には良いことだ。
Sometimes it's good to step off the rails in life.
In this context, 'レールに乗る' implies following the planned or expected course effectively, extending the metaphorical use of 'レール'.