A train was a grouping of vehicles that move over rails, transporting passengers or freight. Trains were widely used on Earth during the 19th, 20th, and 21st centuries. Trains consisted of a locomotive, which provided the power for trains to move, which was attached to one or more vehicles designed to either carry freight or passengers.
During the 19th and 20th century, the Orient Express ran from Paris, France to Istanbul, Turkey. The Orient Express became associated with intrigue and luxury travel. Dr. Beverly Crusher became fascinated with the experience of traveling on the train, and had a holodeck program that recreated the early Orient Express. (TNG episode: "Emergence")
The popularity of using trains for transporting passengers declined during the 20th and 21st centuries as other means of transportation - such as automobiles and air travel - became more popular. Despite this, some humans continued to prefer trains. Zefram Cochrane did not like flying, and preferred trains when traveling. (TNG movie: Star Trek: First Contact)