I sometimes hear people repeat a myth that there is a blanket "no doubling back" rule
In reality, while there are rules about where you can or can't double back, there is certainly no blanket rule against it.
Here are some examples where doubling back is allowed:
Reading West to Bruton
example fare:
example itinerary:
The origin Routeing Point (RP) is Reading Group. Although there is a clause in the Routeing Guide which does not allow doublebacks within the origin/destination routeing group, in this case we are doubling back within the same fares group (fares groups and routeing groups are not the same thing, though in some cases they may share the same membership) which takes priority, so the Routeing Guide check applies to Reading to Bruton, with preceding stations within the same fares group not needing to be considered for this check.
The destination Routeing Point for this journey is Castle Cary, which is an appropriate routeing point for Bruton in this case. There is no doubling back within the maps (in this case the relevant map is WE). Castle Cary to Bruton is a valid local journey from the destination. Again, providing there is no double-call at Bruton, the fact that this is a double-back for the overall journey is not relevant.
Cambuslang to Watford Jn
example fare:
example itinerary:
As the ticket has a maltese cross, we can do the split route check to/from London Group. This means all permitted routes to London (in this case we are simply using map LM for that purpose) and all permitted routes from London (map EN in this case).
Although the customer would be entitled to use a multitude of routes which do not require a double-back (e.g. via Birmingham, Reading and Paddington), there is absolutely nothing preventing them from doubling back between Watford Jn and London Euston as the split route check has no doubling back on each of the component legs. The rules allow for it.
Cheadle Hulme to Watford Jn
example fare:
example itinerary:
Again, valid. The origin RP is Cheadle Hulme itself, however the double-back to Stockport is covered by easement 700074.
As the fare is routed +via London, this means it is valid via London (as well as on any cheaper flows e.g. route Not via London).
As we are travelling via London, we again have to do the split route check. We therefore take all permitted routes to London (in this case map EO) and all permitted routes from London (in this case map EN). If the split route check results in a double-back for the overall journey then so be it; there is absolutely no rule against it providing each component leg is valid.
Note however that the routeing does not compel passengers to travel via London. It is entirely legitimate to avoid the double-back and alight from a southbound train at Watford Jn. It would also be legitimate to break their journey at a valid location eg. Harrow & Wealdstone, and when they resume their journey there is no compulsion to do the double-back so they could proceed directly to Watford Jn.
However booking engines cannot handle this extended validity; they will only issue such a ticket if the route does actually go via London. See also:
https://www.railforums.co.uk/threads/london-euston-to-edale-via-stockport.148525/ for another example of this, with a "via Manchester" fare; the passenger is permitted to double-back between Stockport & Manchester, and booking engines will only issue this fare for the itinerary concerned if the customer obtains an itinerary via Manchester however the passenger is not actually compelled to complete the double-back between Stockport and Manchester.
I hope that provides some clarification!
In reality, while there are rules about where you can or can't double back, there is certainly no blanket rule against it.
Here are some examples where doubling back is allowed:
Reading West to Bruton
example fare:
example itinerary:
Reading West 1427
Reading 1433
Reading 1505
Castle Cary 1640
Castle Cary 1707
Bruton 1712
This is valid. The fact you double-back between Reading West & Reading is indisputably valid providing you do not double-call at Reading West.Reading 1433
Reading 1505
Castle Cary 1640
Castle Cary 1707
Bruton 1712
The origin Routeing Point (RP) is Reading Group. Although there is a clause in the Routeing Guide which does not allow doublebacks within the origin/destination routeing group, in this case we are doubling back within the same fares group (fares groups and routeing groups are not the same thing, though in some cases they may share the same membership) which takes priority, so the Routeing Guide check applies to Reading to Bruton, with preceding stations within the same fares group not needing to be considered for this check.
The destination Routeing Point for this journey is Castle Cary, which is an appropriate routeing point for Bruton in this case. There is no doubling back within the maps (in this case the relevant map is WE). Castle Cary to Bruton is a valid local journey from the destination. Again, providing there is no double-call at Bruton, the fact that this is a double-back for the overall journey is not relevant.
Cambuslang to Watford Jn
example fare:
example itinerary:
Cambuslang 1109
Glasgow Central 1122
Glasgow Central 1140
London Euston 1609
London Euston 1630
Watford Jn 1646
This is also valid. The origin RP is Glasgow Group. The fact that Cambuslang lies on the Glasgow-London main line is not relevant. Cambuslang-Glasgow is a permitted local journey. We then use the mapped routes from Glasgow to the destination RP.Glasgow Central 1122
Glasgow Central 1140
London Euston 1609
London Euston 1630
Watford Jn 1646
As the ticket has a maltese cross, we can do the split route check to/from London Group. This means all permitted routes to London (in this case we are simply using map LM for that purpose) and all permitted routes from London (map EN in this case).
Although the customer would be entitled to use a multitude of routes which do not require a double-back (e.g. via Birmingham, Reading and Paddington), there is absolutely nothing preventing them from doubling back between Watford Jn and London Euston as the split route check has no doubling back on each of the component legs. The rules allow for it.
Cheadle Hulme to Watford Jn
example fare:
example itinerary:
Cheadle Hulme 1324
Stockport 1329
Stockport 1343
London Euston 1539
London Euston 1554
Watford Junction 1609
Stockport 1329
Stockport 1343
London Euston 1539
London Euston 1554
Watford Junction 1609
Again, valid. The origin RP is Cheadle Hulme itself, however the double-back to Stockport is covered by easement 700074.
As the fare is routed +via London, this means it is valid via London (as well as on any cheaper flows e.g. route Not via London).
As we are travelling via London, we again have to do the split route check. We therefore take all permitted routes to London (in this case map EO) and all permitted routes from London (in this case map EN). If the split route check results in a double-back for the overall journey then so be it; there is absolutely no rule against it providing each component leg is valid.
Note however that the routeing does not compel passengers to travel via London. It is entirely legitimate to avoid the double-back and alight from a southbound train at Watford Jn. It would also be legitimate to break their journey at a valid location eg. Harrow & Wealdstone, and when they resume their journey there is no compulsion to do the double-back so they could proceed directly to Watford Jn.
However booking engines cannot handle this extended validity; they will only issue such a ticket if the route does actually go via London. See also:
https://www.railforums.co.uk/threads/london-euston-to-edale-via-stockport.148525/ for another example of this, with a "via Manchester" fare; the passenger is permitted to double-back between Stockport & Manchester, and booking engines will only issue this fare for the itinerary concerned if the customer obtains an itinerary via Manchester however the passenger is not actually compelled to complete the double-back between Stockport and Manchester.
I hope that provides some clarification!
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