Travelling Post Offices, or TPO’s, were trains that were equipt with letter boxes, so that mail could be posted while the train stood idle in a station.
A feature of the TPO was the picking up and dropping off of mail, without stopping. Special line side apparatus was created to enable this transaction. At the line side, mail was suspended in strong leather pouches and a net could be extended from the carriage to catch these bags and project them back inside. The mail was then sorted within the carriage. To dispatch mail, a hinged bracket on the side of the coach enabled the post to be deposited into heavy-duty line side nets. You can find more information about this here.
Fragile letters, that could not withstand the use of line-side apparatus, were withdrawn and send via other routes.
As seen in the item illustrated, special labels were occasionally applied to explain this withdrawal, stating, “This Packet has been diverted from the usual route, as it appeared to be unsuitable for transfer by the Mail Apparatus.” Crossed blue lines can be used to illustrate this as well.
You can find this item paired with another on our website, here.