mishap vs derailment

mishap and derailment both are nouns.

Word	Noun	Adjective	Verb	Adverb
mishap	Yes	No	No	No
derailment	Yes	No	No	No

As nouns, derailment is a hyponym of mishap; that is, derailment is a word with a more specific, narrower meaning than mishap:

Other hyponyms of mishap include accident, near miss, ground loop, puncture, slip, trip, crash.

mishap (noun)	derailment (noun)
an instance of misfortune	an accident in which a train runs off its track
an unpredictable outcome that is unfortunate