EUROPE.

SWITZERLAND.

Anything down Lake Geneva or special trains like the 'Glacier Express'.

NORWAY.

Bergen to Oslo is well-known for many reasons - it's stunning.

RUSSIA/ASIA.

THE TRANS-SIBERIAN/MONGOLIAN.

Moscow to (generally) Beijing, passing through some of the worlds most remote places for days on end.

AFRICA.

THE BEST ROUTE.

Dar es Salaam, Tanzania) to (almost) Lusaka, Zambia. Pass through pristine African bush for two days and a night - better value than the raved about Nairobi to Mombasa line. Get off at Mbeya to travel Malawi. 

FOR THE ADVENTUROUS.

Dakar (Senegal) to Bamako (Mali): an out-of-this world 30-36 hours - very hard work, may leave you mentally scarred!

SOUTH AMERICA.

HIGH ALTITUDE MOONSCAPES.

Calama (Chile) to Uyuni (Bolivia)

Honorary mentions.

Any train journey in India or China, Japan's Shinkansen (bullet) trains which with a rail pass are quite affordable, without - ridiculously expensive.