I have just finished watching the movie 'Murder on the Orient
Express', having not read the Agatha Christie novel.

However, I was frustrated and dissapointed at the ambiguous nature of
the ending, as I still don't know how the murder was committed.

My question is thus: Who really committed the murder - was it a
conspiracy of all the passengers as considered by Pirot, or was it the
single person that dissapeared, as agreed at the conclusion of the
movie?

"Murder on the Orient Express" is one of my favorite Agatha Christie
films, so I'm glad to have an opportunity to give my viewpoint.

Hercule Poirot is asked to investigate an onboard murder by his
friend, Bianchi, who works for the train company that operates the
Orient Express. The investigation leads to two theories: that the
murder was committed by an unknown person who has fled, leaving a
train conductor's uniform behind, and that there was a conspiracy
involving a group of people, each of whom had a motive to kill the
victim. The members of the group each stabbed the victim once.

The conspiracy theory is most likely to be accurate, but there were
twelve stab wounds, and Poirot knows that there is no way to prove
exactly who struck the killing blow. Poirot presents both theories to
Bianchi, and lets Bianchi decide how to handle the situation. Bianchi
chooses the single killer theory. It is likely that Poirot himself
believes the conspiracy theory to be true.

We, the audience, are free to believe as we will, but I think most
viewers are likely to side with the multi-killer theory, particularly
since we are shown the details in "flashback" style as if they are
actually happening.

I hope this helps! Thanks for an enjoyable question.

Best regards,
pinkfreud

Thanks for your excellent summary and reasoning. I'll go with the
multi-killer conspiracy theory - especially given the toasting of
glasses at the end.

From my raddled memory - it was all of them.

Although technically you cannot 'murder' a corpse, or someone who is
fatally injured.