: The author closest to the source is the best bet. :
Except when they are wrong...
TPOs with their off-set gangways was due to internal mail sorting equipment layout, any fool can see that just by examining the internal layout of such coaches, some things simply do not change against physical facts how ever you or a miss-guided author wishes, the size of letters (aka the size needed for sorting racks, the average space taken up by a standing or sitting postal worker, the size of a postal sack etc.
Also, you have never explained why a simple key-lock was deemed totally adequate for external door security but not for the corridor connection doors and thus requiring them to be off-set - with all the complications that introduces, even within just the specialised world of TPO stock!