When and where were TPOs used?
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The first TPO ran in January 1838. It travelled on the Grand Junction Railway between Birmingham and Liverpool. It was called the 'Railway Sorting Carriage' and consisted of a converted horse box fitted out to allow mail to be sorted on board. George Karstadt, the man who had set up the trial, saw his son work as one of the first mobile sorters.

This successful experiment saw other routes quickly set up. On 17 September 1838, the London and Birmingham Railway opened throughout its length and TPOs were established on the line, looking like the one in the picture above. Two despatches left London Euston each day: an 11.00am 'day mail' and 8.30pm 'night mail'.
Routes
were marked as either 'up' or 'down' depending on their direction of
travel. The 'up' line headed towards London while the 'down'
line headed away. The picture below is from the Great Western Down Special in 1980, next to four TPO postmarks from around the country.

Foreign
mail was also carried by TPOs. 'Continental mail' was carried between London and Dover,
and the 'Irish mail' connected with the Holyhead to
Kingston Packet Boat Service.
Timing is everything
On all
TPO routes, timing was crucial and delays were disastrous causing problems with
deliveries throughout the whole postal network. Timebills were prepared for each route and any deviation from the times
on the bill had to be recorded.
In the 19th century, there was no national standard time until 1880. The mail guard would carry an official timepiece set to 'London time' with which he could record arrival and departure times. It was well known that a country dweller could set their clock by the arrival of the TPO.
The picture on the right shows the handover of the official timekeeper watch from the TPO officer in charge to a postman. The postman would in turn hand it to the officer in charge of the steam packet service across the Irish Sea. This system of carrying an official watch dated from the days of the horse-drawn mail coaches.