I quite agree that easements, negative or postive, do not apply to passengers travelling on through trains or by the shortest route, as they are valid by clause 13 of the conditions of carriage and hence need not consult the routeing guide, let alone the easements.
I also believe that negative easements in general are unenforcable as the routeing guides instructions do not allow for them. The only mention of easements in the instructions is to allow long routes where both the origin and destination have a common routeing point, and also to allow doubling back where it would not normally be permitted. Thus if a passenger, following the instructions of the routeing guide, finds that their intended route would appear to be permitted, they would have no reason to check the list of easements. A passenger would only ever look at the easements as a last resort if the other rules deem there intended route invalid.
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Michael, I obviously do believe that breaking a journey does involve a change of trains. My reasoning that a break of journey is valid on such a journey as Peterborough-Liverpool St via Ipswich is that the break of journey is a result of a choice made by the passenger, hence the change of trains is an optional choice by the passenger, hence the change of trains is not neccessary as it would be possible to complete the journey on one advertised through train. The use of the word may in this part of the routeing guide does not in any way influence my thinking.
Also, note the use of the plural in the line:
"Most customers wish to make journeys by through trains or by the
shortest route. In both cases they will be travelling on a permitted route,"
http://www.atoc.org/clientfiles/File/RSPDocuments/instructions.pdf
thus implying that it is possible to travel on more than only one through train, which is only possible if a break of journey is made.
On a similar note, I do concede that the conditions of carriage state that "a through train", ie only one, may be used, but this doesn't matter because you then refer to the routeing guide as per clause 13.a.iii and such a break of journey on a through train is permitted.