All were elected by the Chapter and installed by the bishop. Saint-Nicolas-au-Cloître d'Amiens also had a Chapter, composed of a Dean and eight prebendaries, all elected by the Chapter and installed by the bishop. The church of St. Acheul, near Amiens, and formerly its cathedral, was, in the 19th century, the home of a major Jesuit novitiate. The beautiful churches of St. Ricquier and Corbie perpetuate the memory of the great Benedictine abbeys and homes of learning founded in these places in 570 and 662. In 859 the Normans invaded the valley of the Somme, and sacked the abbey of Saint-Riquier. They pillaged Amiens and held it for more than a year, until the city was ransomed by Charles the Bald. There is a medieval list of the Bishops of Amiens, but it first appears in the work of Robert of Torigni in the second half of the 12th century, and its names before the 8th century are very uncertain. London Underground engineering stock  Over the years, London Underground has acquired various types of engineering stock to help with the construction of new lines and maintenance of existing lines. Some of these wagons were inherited from its predecessors, many were built new and some were acquired second-hand from the main-line railways. Several types of specialist wagons have been used, which are described below. London Underground uses a numbering system comprising the wagon number prefixed with a letter which designates the wagon's type. These are summarised below. London Underground has used various designs of brake van. They were used for the guard's accommodation and for braking purposes (when wagons in the train were not fitted with automatic brakes). Two brake vans of interest were numbers FB578 and FB579. These were 'flat' brake vans, converted from flat wagons. The conversion consisted of building a small guard's compartment on one end of the flat wagon. Therefore, these wagons could be used for carrying small items, such as sleepers, in addition to their role as a brake van. Six brake vans numbered B580–5 were built for London Transport by British Railways (BR) at the latter's Ashford Works in the early 1960s. They were built to a design which had been produced for BR since 1950 (BR Diagram 1/506 and 1/507), totalling 3588 units, and were the final examples of that design to be constructed.