Shortest route + 3 Mile rule - inconsistent results across booking engines

[JB_B]

I've been taking a quick look at how booking sites handle the three mile rule.

I tried six non-mapped journeys which don't go via the shortest route but which I think should be valid because they are within 3m of the shortest route.

A: Liverpool Lime Street to Aigburth via Liverpool South Parkway SDS 3.20 (from Liverpool Stations)

B: Queens Park Glasgow to Pollokshaws East via Kings Park Glasgow SDS 2.10

C: Crystal Palace to Norwood Junction via Sydenham SDS 2.50 (route: Not London)

D: Penzance to Hayle via Lelant Saltings SDS 3.80

E: Cradley Heath to Kidderminster via Stourbridge Town SDS 3.90

F: Bangor to Rhyl via Deganwy SDS 14.00


Trying to find departures on 7 November at 06:00 gives

GWR:-
A-Yes
B-Yes
C-Yes
D-No
E-No
F-No


NRE:-
A-Yes
B-Yes
C-Yes
D-No
E-No
F-No

Southern Railway:-
A-Yes
B-Yes
C-Yes
D-No
E-No
F-No

Trainline:-
A-Yes
B-Yes
C-Yes
D-Yes
E-Yes
F-Yes

Trainsplit:-
A-Yes
B-Yes
C-Yes
D-Yes
E-Yes
F-Yes


The obvious factor connecting routes D,E,F is that they involve passing through the same station twice (which, as far as I know, isn't prohibited for these local journeys).

Could it be that the sites that reject D,E,F are incorrectly applying a no double-back rule - or is something else going on?

 

[krus_aragon]

Looking at F (Bangor-Deganwy-Rhyl), the doubling back at Llandudno Jn is what came to mind.

I'm also reminded that Llandudno Jn only became a routeing point (as opposed to all NWCL stations being associated with Shotton) because of the anomaly that if your outbound train didn't stop at Rhyl, it'd be valid for you to continue to the routeing point (Shotton) and then take a stopping train back to Rhyl. I'm not sure if these scenarios are connected.

It does strike me as an odd itinerary, as you'd have to change trains at the routeing point in order to achieve that double back, and force at least two changes on what would otherwise (usually) be a direct journey. The only benefit I can think of is diverting to visit your Aunt Gladys in Deganwy. I can't find anything to specifically rule it as disallowed, though.

 

[Harpers Tate]

Is the three mile rule still present in the routing guide? I had a feeling it had quietly disappeared.

 

[krus_aragon]

Is the three mile rule still present in the routing guide? I had a feeling it had quietly disappeared.

It's still on page 1 and 7 of "the national routeing guide in detail", but there's no meniton of it in the routeing guide " instructions".

 

[JB_B]

Looking at F (Bangor-Deganwy-Rhyl), the doubling back at Llandudno Jn is what came to mind.

I'm also reminded that Llandudno Jn only became a routeing point (as opposed to all NWCL stations being associated with Shotton) because of the anomaly that if your outbound train didn't stop at Rhyl, it'd be valid for you to continue to the routeing point (Shotton) and then take a stopping train back to Rhyl. I'm not sure if these scenarios are connected.

That's right, D,E and F all deliberately involve double backs. I don't think the routeing point status is an issue since they are purely local journeys in the sense that there are no mapped routes.

It does strike me as an odd itinerary, as you'd have to change trains at the routeing point in order to achieve that double back, and force at least two changes on what would otherwise (usually) be a direct journey. The only benefit I can think of is diverting to visit your Aunt Gladys in Deganwy. I can't find anything to specifically rule it as disallowed, though.

Yes, they're constructed to explore the sites' behaviours and few passengers would actually wish to go that way. ( I guess being able to break at Town rather than Junction when going through Stourbridge is the most likely to be of practical use.)

 

[krus_aragon]

That's right, D,E and F all deliberately involve double backs. I don't think the routeing point status is an issue since they are purely local journeys in the sense that there are no mapped routes.

Looking again, I see the two issues are in adjacent sentences on page 7:
"If there is a common routeing point, the permitted route is the shortest route or a
route which is longer by no more than 3 miles."
and
"Also permitted is the route followed
by direct trains to and from the common routeing point if the journey is made on
those trains."

The latter is the sentence that gave rise to journeys like Bangor-Rhyl via routeing point Shotton. As it was an unintended (but technically correct) interpretation of the routeing guide, Llandudno Jn (and Pontypridd, among others) became routeing points to avoid this, and the size of the routeing guide exploded as a result.

The double-back on a shortest+3 route is probably equally unintended, but I can't see how it could be indirectly removed without changing the rules, as was done above.

I susptect that both interpretations were disallowed in some circumstances by certain journey planners' broad application of the no double-back rule.