Title error
It's a stretch to consider these two strangers. Granted Guy never met Bruno but he was NO stranger to ANYBODY.
The premise is to put forth the notion 2 strangers can meet, discuss problem and agree to act and hope that the fact they are strangers no one will commit to memory seeing them together.
But Guy is a celebrity. People would CERTAINLY remember him on the train when he was initially talking to Bruno or anybody for that matter.
So the set up would not be so surprising.
This error is played on later when the drunk can't remember sseing Guy on a later train. This tells me Hitchcock new this and was again, playing with us.