In the early 1870s, several companies, both in the US and the UK, started to use bogie carriages. All of these carriages were based on wooden frames and thus at risk of serious damage in a collision. In 1872 one company came up with the radical idea of having a wrought iron chassis for the carriage, and extending the iron structure to include the body of the carriage as well. Two carriages, numbers 15 and 16, were built to this design, and entered service on the Ffestiniog Railway.
In late 1872 the first body was delivered from the Birmingham firm of Brown, Marshalls and Co. Ltd., of Adderley Park, Birmingham, the details being their own, but with an overall general design by Charles Easton Spooner. Whilst not the world's first, they were the first revenue-earning bogie coaches in the British Isles, and are now the oldest in existence.
Other railways rapidly adopted the idea of metal chassis and bogie carriages, but it was not until 1964, nine decades later, that British Rail finally moved to having an integral chassis and body frame. Almost every carriage on Britain’s main line railways now has this feature – which started on the Ffestiniog, where you can still ride in the original 15 and 16 to this day.