With the help of Railway Codes, the wonderful resource that holds all the “Engineer’s Line References,” some sense of the Plymouth mileage change has been made, which used to be – or may still be – marked by a board on Cornwall Loop.
The distance from North Road Junction (east of the former triangle) to Millbay and back to Cornwall Loop Junction was one mile 34 ch. From North Road Junction direct to Cornwall Loop Junction is 21 chains. The discrepancy in the modern main line is therefore one mile 13 ch.
North Road Junction was 246 m. 8 ch.; Cornwall Loop Junction was 246 m. 29 ch. A point between them for some reason was chosen as the mileage “junction.”
The line starts seven chains from the original blocks at Paddington. The new line from Wearde is two chains longer than the old one. Another two chains are gained between Baldhu and Chacewater. By the scribe’s reckoning, altogether, that makes Penzance, not 326 m. 50 ch., but 325 m. 34 ch. But he’ll happily be corrected.
He’s not going to look at the Berks & Hants.
The Great Western was one of only two companies, pre-grouping, to have a milepost 300. The other was not the London & North Western, as may easily have been thought, but the Midland, whose route to Carlisle was slightly longer. St. Pancras to Petteril Junction is 307 m. 14 ch.