Starting in the late 19th century, an increasing number of Britons began travelling abroad due to the advent of railways and travel services such as the Thomas Cook Continental Timetable. The speed of trains, as well as the number of passengers that crossed multiple borders, made enforcement of passport laws difficult, and many travellers did not carry a passport in this era. However, the outbreak of World War I led to the introduction of modern border controls, including in the UK with passage of the British Nationality and Status of Aliens Act 1914. Thus, in 1915 the British government developed a new format of passport that could be mass-produced and used to quickly identify the bearer. The new passport consisted of a printed sheet folded into ten and affixed to a clothed cardboard cover. It included a description of the holder as well as a photograph, and had to be renewed after two years.