What does the cryptic crossword clue "Lighter trains, so going off the rails? (8)" mean?

The definition is "Lighter". The wordplay uses an anagram. anagram of TRAINS, + SO = ARSONIST; definition: "Lighter" The answer is Arsonist.

Lighter trains, so going off the rails? (8)

Telegraph Prize Cryptic #3003 | 18 Across | 2019-05-12 flower