I watched the 2017 Murder on the Orient Express film last night. After
is murdered, we find out that
he was actually Casetti
But how did Poirot know that?
At the time, it seemed like Poirot simply announced that that's who he was, and then started using that information to untangle the plot.
In particular, IIRC Poirot knew this before he found and filled in the partially burned note.
If I've merely misremembered the sequence of events, and he actually found the note first, then the letters he filled in to find Daisy Armstrong's name seems too implausible a leap, if there has been no prior link to the Armstrong case.