Have you any idea how far you can travel in train without any other mode of transport.?
Yes by only train.
OK the answer is almost 17000 km. 17 thousand km..!!!! Or 10 thousands miles.
From Portugal to Vietnam you can travel solely by train. And this is the longest possible route for travel by train only.
Try it and inform me how’s the journey ?
I have no plan to try that because it would take almost 1 month. But there are always some crazy people who would do anything for adventure. In 2008 Michael Hodson and along with other 2 crazy blogger took the The Ultimate Train Challenge and traveled from Lisbon, Portugal to Saigon, Vietnam. Total fees was for ticket in train cost less than 1000 $.
Google map will show you the entire track details. Try and search and also follow this website for more details about this thrilling travel experience

Photos of Longest Train Journey