derailment — when a train comes off its tracks; the physical event of leaving the rails.
早朝、貨物列車が脱線して線路が一時不通になった。
In the early morning, a freight train derailed and the track was temporarily blocked.
脱線事故の原因は現在捜査中だ。
The cause of the derailment is currently under investigation.
digression; deviation — when a conversation or discussion strays from the main topic.
会議中に脱線して、本来の議題に戻れなくなった。
The meeting went off on a tangent and we couldn't get back to the original agenda.
話が脱線しないよう、司会が議題を整理した。
To prevent digression, the moderator organized the agenda.