Investigator came down and stepped on to platform? (9)

I believe the answer is:

detrained

I'm a little stuck... Click here to teach me more about this clue!

'stepped on to platform?' is the definition.
I can't tell whether this defines the answer.

'investigator came down' is the wordplay.
'investigator' becomes 'det' (I can't explain this - if you can you should believe this answer much more).
'came down' becomes 'rained' (raining is a kind of coming down).
'det'+'rained'='DETRAINED'

'and' is the link.

Can you help me to learn more?

I've seen this clue in The Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.