Consensus of opinion is not proof of what really happened. But by indirect means, when the impossible has been eliminated, and only the highly improbable remains, then that points to the most logical conclusion.
What is missing is the WHO and HOW, two very important sets of facts that have been intentionally obliterated or seriously muddled, so as to make the perpetrator much less apparent.
Means, motive, and opportunity. Keep hammering, and eventually, somebody will crack.
Plenty of people had the motive.
Murder on the Orient Express