Land crossings are the best even though I love flying!

By Train:
Finland -> Russia
Russia -> Mongolia
Mongolia -> China
China -> North Korea
Germany -> Switzerland
Switzerland <-> Italy
Italy -> France
Luxembourg <-> France
France <-> Monaco
UK -> France
France -> Belgium
Belgium -> Germany
Slovenia -> Croatia
Croatia -> Serbia
Serbia -> Macedonia
Russia -> Ukraine
Ukraine -> Moldova
Moldova -> Romania
Romania -> Bulgaria
Bulgaria -> Turkey
Thailand -> Malaysia

By Car/Bus:
Armenia -> Georgia
Germany <-> Austria
Austria <-> Hungary
Togo <-> Benin
Poland -> Slovakia
Slovakia -> Czech Republic
Czech Republic -> Poland
Estonia <-> Latvia
Latvia <-> Lithuania
Russia -> Finland
Finland <-> Norway
Finland <-> Sweden
Jordan -> Syria
Syria -> Lebanon
Kenya <-> Tanzania
Switzerland <-> Liechtenstein
United Arab Emirates <-> Oman

On Foot:
Ghana <-> Togo
Togo <-> Ghana
Ecuador <-> Peru
Italy <-> Vatican

By Ferry/Boat:
Sweden <-> Denmark
Denmark <-> Germany
Brunei <-> Malaysia
Finland <-> Sweden
Finland <-> Estonia
China (Hong Kong) <-> Portugal (Macau)