This article is a travel topic
Since the 1960s overlanding has been a popular means of travel between destinations across Africa, Europe, Asia (particularly India), the Americas and Australia. The "Hippie Trail" of the 60s and 70s saw thousands of young westerners travelling through the Middle East to India and Nepal. See Istanbul to New Delhi over land and Europe to South Asia over land.
At 9,288km the Trans-Siberian Railway is one of the longest overland journeys in existence today, taking 7 days to reach Vladivostok from Moscow, and providing an alternative to air travel for journeys between Europe and Asia.
In USA Amtrak can carry you overland from New York across the continent to San Francisco on the Pacific Coast and back and forth across the continent using different routes, just a part of their 21,000 mile train line system
The introduction of Japan's high speed railway Tōkaidō Shinkansen in 1964 changed the face of rail travel. The railway has carried more than 4 billion passengers and its new N700 series trains are capable of 300km/hr. France's TGV attains similar speeds, making it faster than air travel for many journeys. It has expanded into other European countries. Korea and the USA now also have some high-speed lines. China is in the process of building what will be the world's largest such network; see High-speed rail in China.
Buses and Overland Safari Trucks
The Silk Route or Silk Road historically connects the Mediterranean with Persia and China. Today the route refers to overland journeys between Europe and China, taking either the northern route - through Russia and Kazakhstan - or the southern route - through Turkey, Iran, Pakistan and North India - to Urumqi or Xian in China. These routes are still popular today, with companies such as Oasis Overland  and Odyssey Overland  offering tours on the southern route.
Trans Africa Overland routes
Some of the longest and more traditional overland routes are in Africa. The Cairo to Cape Town and v.v. route covers more than 10,000km, usually following the Nile River through Egypt and Sudan, continuing to Kenya, Tanzania, Malawi, Zimbabwe, Botswana and Namibia along the way. From the mid 1980s, the non-operation of the Aswan to Wadi Halfa ferry between Egypt and Sudan as well instability in Sudan, northern Uganda and Ethiopia, made the journey impossible. In recent years however, the Cape to Cairo and Cairo to Cape Town route has again become possible and increasingly popular both with commercial overland trucks carrying groups of 20 or so paying passengers as well as independent travellers on motorbikes or with 4WD vehicles.
The traditional Trans Africa route is from London to Nairobi, Kenya and Cape Town, South Africa. The route started in the 1970s and became very popular with small companies using old Bedford four wheel drive trucks carrying about 24 people each, plus lots of independents, normally run by groups of friends in 4x4 Land Rovers heading out of London from November to March every year. The usual route was from Morocco to Algeria with a Sahara desert crossing into Niger in West Africa, continuing to Nigeria. This was followed by a month long journey likened to Joseph Conrad’s “Heart of Darkness” through the forests of Zaire (now Democratic Republic of Congo), surfacing into the relatively modern world in Kenya via Uganda. From Kenya the last leg was south through Tanzania to either Zimbabwe or South Africa.
This route has changed dramatically due to border closures and political instability creating no-go zones. The route has reversed itself somewhat over the last few years, with trucks now crossing from the north to the south of Africa, closely following the west coast all the way from Morocco to Cape Town with the biggest change in the route being made possible by the opening of Angola to tourism. The journey then continues through Southern and East Africa from Cape Town to Nairobi and on to Cairo.
In south Africa the most popular route is Cape Town to Victoria falls. Companies have trucks leaving daily starting the route in Cape Town South Africa. The Cape to Vic route takes travellers through Namibia, Botswana and ending in either Zimbabwe or Zambia. 
Since 2006 a few companies have offered overland expeditions from the UK to Australia. Originated by Exploratory Overland Expeditions  in 2006, the expedition is marketed as the longest trans-Asian overland journey available.
The longest overland expedition of any kind is run by African Trails  their London-Capetown-Istanbul journey (43 weeks) remains the classic overland expedition for die-hard travellers.Though the longest combination of trips is 50.5 weeks run by Dragoman from Helsinki, Finland to Cape Town, South Africa via Russia, China, Middle East, following the Nile and to Kenya and on to southern Africa.
These days’ overland journeys explore most continents the only one not yet travelled is Antarctica, Overlanding companies such as Flatdog Adventure , make overlanding a fantastic next step from a fully inclusive hand held tour to that of full independent travel.