I would like to come forward with a unique route suggestion. I believe Greece would be a very good country for a route. Be it the Suburban railway which goes from the Piraeus Port to the Airport, or be it the long line from Athens to Thessaloniki. Either way, if these route don't sound interesting, there is always the scenic railway of Diakopto - Kalabryta which is a route that happens to pass through mountains to reach a village on a hill. I would also like to add that the Suburban railway has many more routes, like the one that stretches to Xalkida or the one that stretches to Kiato (with an expansion to Patras coming soon). If you are instead looking for an underground route to go along-side with the Bakerloo line, Athens has 2 underground metro lines them being, Line 2 (Helliniko - Anthoupoli) and Line 3 (Dimotiko Theatro - Airport).
I used to work with a couple of guys who did a fair bit of travel around Greece in the 90's when the Alco's were still king. They used to say the line across the Bralos was particularly spectacular.
