Britain's 'Travelling Post Office'
According to the site, "TPO's consisted of one or more rail carriages in which the manual sorting of letters take place. The first experimental TPO travelled on 6 January, 1838 between Birmingham and Liverpool and was made out of a converted horse box. After Uniform Penny Postage was introduced in 1840 and the volume of mail exploded, many more of these carriages were introduced to cope with the demand."
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