Explanation to the logic problem:

John was blind when he went to London. Paul was a doctor who operated on him & cured him - hence his happiness.

On the way back, the train was in darkness in the tunnel, John thought he was blind again, couldn't stand the thought, so killed himself.

Logical, eh? You'll get lots of groans when you tell the students or they eventually discover it through their questions.

There are more 'logic problems' in Challenge To Think' by Frank et al (OUP) although I'm not sure it's still in print.