The bogie was patented by William Chapman in 1813, though its first application in England wasn't until 1833 on a locomotive built by J. and C. Carmichael for the railway at Dundee, Scotland. The bogie has four wheels in pairs, mounted on the bogie frames, which connect to a centered pin descending from the locomotive. Gravity holds bogies on the pins, they are never fastened because that leads to binding on rough or uneven track, which causes derailment. Switch engines usually do not have pilot trucks (or bogies), they can traveled slower through tighter curves than road locomotives.