Something that is more Alex friendly. A murder mystery based on the Orient Express. Could be stopping at various continental locations. Could end in Istanbul?

So how about defined cities, with each one slightly different, and a different event in each that all build up to a thing. Could be an espionage thing rather than murder? Cities might be:

• Paris.
• Somewhere on the Rhine.
• Vienna.
• Venice.
• Budapest.
• Istanbul.

Maybe they uncover a plot to… do what? Break into a bank? Subvert a government? They'd have to collect various bits of information and build the whole picture. There need to be some shadowy people on the train.

Perhaps each of them has a persona? That would be a bit dangerous though.

Maybe… they find a briefcase with a top secret report they need to deliver and people keep trying to stop them? And some spy gadgets, money, passport.

Each carriage compartment has a suspect, or a person of interest. Every day they have to investigate that person. Each one is part of a conspiracy that builds up to…?