Digression
Derailment, Be Derailed
noun, verbal noun
When you undress from the line it's like you're peeling something off the line, going off in another direction, and then causing a digression in whatever the line was about. It's like when you're talking and you digress. Think of your speech as a line. Then, undress a part of it and let it fall away. That is your digression.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
その電車は、運転手のとっさの判断で、脱線をまぬがれました。
The train was able to avoid derailment because of the driver's quick decision.
話が脱線しました。
I digressed from the subject.
電車が脱線しましたが、怪我人はありませんでした。
The train was derailed, but nobody was injured.