The passing of the Railways Conveyance of Mails Act in 1838 required railway companies to carry mail, by ordinary or special trains, as required by the Postmaster General. TPOs were employed in many British Commonwealth countries, and the Army Post Office had its own TPOs. Effectively, this was a system of constantly moving depots that allowed for mail to be constantly on the move, fed by the immensely complex system of tracks across the country. This was vital as road transport had not yet developed enough to become a reliable form of cross-country connection.