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…?

So think of it like Cluedo. There are a set of categories. Each one they need to solve. Each one has a person on the train to investigate. Could be video'd interviews even? That person has evidence that can answer one of the categories. They might not solve them all. But at the end they're asked to give their answer to the secret intelligence service agent in Istanbul. Then they have to give their answer, but they might not know whether it's the right one.

So what might the categories be?

• What are the secrets about?
• Who is trying to obtain them? (could be many - competition between different countries' secret services and they need to find out which?)
• Where is the British agent?

Think John Buchan.

So, every day they arrive at a station and receive a package directing them from the station master that's been telegraphed from London.