History of the Metropolitan Police
'Black Maria' was the nickname for secure police vans with separate locked cubicles, used for the transportation of prisoners. The name is said to have come from a large and powerful black lodging-house keeper named Maria Lee, who helped constables of Boston, Massachusetts in the 1830s when they needed to escort drunks to the cells.
Early Horse Drawn Prison Van
The Met's first vehicle of any kind was a Black Maria drawn by two dray horses, acquired in 1858. Within 30 years the number had increased to eight, and a special area in the yard of Bow Street Police Court was reserved for them to load and unload their charges.