Freighthopping or train hopping is the act of surreptitiously boarding and riding a railroad freight car. In the United States, this became a common means of transportation following the American Civil War as the railroads began pushing westward, especially among migrant workers who became known as "hobos". It continued to be widely used by those unable to afford other transportation, especially during times of widespread economic dislocation such as the Great Depression. For a variety of reasons the practice is less common today, although a community of freight-train riders still exists.
Modern day hopping
Hopping trains happens all over the world and varies from place to place. Some places are more critical and consider freight hopping a crime, and other places are more lenient.
For the USA, a famed "document" detailing know-how of good freight hopping locations and routes is the "Crew Change Guide", rumored to be available only from experienced hoppers. Public access to such information can be problematic for hoppers, since it may increase the dangers and difficulty of their travels. The railroad police, also known as bulls, have become more observant and careful when looking for train hoppers.However, with the spread of online maps, GPS, smartphones and radio such "insider" knowledge has become less important for newcomers and some of the valuable knowledge from 20 years ago now is even obsolete.
From Wikipedia