Case of the Last Train

When this case enters, create two 2/2 white and
blue Detective creature tokens, then investigate.
(To investigate, create a Clue token. It's an artifact
with "{2}, Sacrifice this artifact: Draw a card.")
To solve — Seven or more nonland permanents
entered the battlefield under your control this
turn. (If unsolved, solve at the beginning of your
end step.)
Solved — Tokens you control have hexproof.