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.
The train was able to avoid derailment because of the driver's quick decision.
I digressed from the subject.
The train was derailed, but nobody was injured.