The longest train journey in the world, lasts about 21 days. It starts from Portugal in Europe and ends in Singapore in Southeast Asia.
The train crosses 13 countries with stops for visits, starting from Portugal, Spain, France, Germany, Poland, Belarus, Russia Mongolia, China, Laos, Thailand, Malaysia, Singapore.
Is there a company that arranges such a trip?
This article refers to costs for the trip but it seems that multiple companies work together to make it possible, rather than one travel agency organising it. What a trip it would be!