The TPO system was constrained only by route availability in the same way as the mail coaches were by the roads. Of course the Post Office did not need TPO's on every set of rails anymore than it did coaches on every road, it would not have been economic or expedient.
Initially, most of the TPO services ran out of London in roughly the same route pattern as that provided by the road services with the only significant difference being the West Coast route. In mail coach days the routes to Cornwall and the West Country, Dover, Norwich, Cardiff, Holyhead and Birmingham were of paramount importance but none were as significant as the route to York, Newcastle and Edinburgh along the Great North Road. When the railways were laid the West Coast route assumed a great significance providing access to Birmingham, Liverpool, Manchester, the industrial northwest, Carlisle and Glasgow. The East Coast route, while still very important and serving the same towns and cities, as did the Great North Road, was not embraced with quite the same enthusiasm by the Post Office and the TPO service as the West Coast route. Eventually, and in the heyday of the TPO service, the Up and Down Specials supported by the North West Night Down TPO, traversed the West Coast route. These were without doubt the greatest TPO services to work in Britain.
There were many important subsidiary services running across country to make up all parts of the postal system from short run District Sorting Carriages to three or four coach trains like the Shrewsbury - York service and the Midland TPO. Their history is a long and fascinating one, most of it as yet un-researched and unrecorded.