1A train or railcar coming off its tracks; the accident itself.
大雨の影響で貨物列車が脱線した。
A freight train derailed due to the heavy rain.
その脱線事故でけが人が出た。
There were injuries in that derailment accident.
ジェットコースターが脱線するという設定の映画を見た。
I watched a movie premised on a roller coaster derailing.
2A conversation or discussion straying from its intended topic and heading off in an unrelated direction.
授業中に先生の話がよく脱線する。
During class, the teacher's talk often goes off on tangents.
雑談で話が脱線してしまい、会議の時間が足りなくなった。
The conversation went off track into small talk, and we ran out of meeting time.
脱線ばかりせず、本題に戻りましょう。
Let's stop digressing and get back to the main topic.