How do journey planners choose where to have you change?

[whole diary]

[grahame]

Travelling north tomorrow on the 07:20 from Melksham, connecting into the Newcastle train in Gloucestershire.  Due to the closure at Bristol Parkway, the connection between the trains can be made at Gloucester or at Cheltenham.

GWR^▸ (Great Western Railway) web site suggests a change at Gloucester (there from 08:48 to 09:01) and that involves a transfer via the footbridge from platform 2 to platform 4.

Alternative is to change at Cheltenham Spa - arrive 09:04 and leave 09:12.  Same platform (and that cannot change due to the layout!)

I will have a very heavy bag.  And be travelling with a gentleman who's not as nimble on his feet as he once was, also with luggage.  So we will change at Cheltenham Spa.  Why on earth would the journey planner advise anyone to change at Gloucester? I can't see the advantage in so doing.

[ChrisB]

Whats the minumum change time at CNM^» (Cheltenham Spa - next trains)? Might you be within that?

[ChrisB]

Also, if your train is late, they might let the connecting train go in front, but might hold it until you arrive?

[Western Pathfinder]

Hope you both have a good trip,good luck north of the border.

[ellendune]

Why on earth would the journey planner advise anyone to change at Gloucester? I can't see the advantage in so doing.

There are - or at least have been a few times when Gloucester is the best point to change - particularly if changing onto a Cardiff XC^▸ (Cross Country Trains (franchise)) train that sometimes leaves while the GWR^▸ (Great Western Railway) train is changing ends. 

[Surrey 455]

I often find with multi transport journey planners that they each have their own ideas about how to get to different places. I mostly use Google maps, Traveline South East and sometimes National Rail enquiries. Often I will take a journey that involves changing trains along a stretch of line where both trains call at all 3 stations (Leatherhead, Ashtead and Epsom) and whilst the planners might agree on both trains to catch, they will suggest changing at different stations.

Grahame - Traveline South East also offer a walking speed mode which may help. I usually select fast walking in normal circumstances and slow if I have a suitcase. This may give you more time for connections, though I suspect it will be clueless when it comes to the availability of lifts, escalators etc.

[Worcester_Passenger]

The usual choice algorithm works on the basis of maximising the interchange time. So if you're on a 13-minute change at Gloucester and an 8-minute change at Cheltenham, it'll go for Gloucester.

[grahame]

Hope you both have a good trip,good luck north of the border.

Thank you - and thanks everyone for the comments / thoughts / suggestions.  If on time into Gloucester and crew available to take our train on to Cheltenham Spa, we'll change at the latter.  I suspect Cheltenham is a 5 minute official change ... we have longer and the physical layout forces a same platform change anyway.

The usual choice algorithm works on the basis of maximising the interchange time. So if you're on a 13-minute change at Gloucester and an 8-minute change at Cheltenham, it'll go for Gloucester.

Ah - right. No consideration of the relative ease of the stations then 

[Richard Fairhurst]

I will have a very heavy bag.  And be travelling with a gentleman who's not as nimble on his feet as he once was, also with luggage.  So we will change at Cheltenham Spa.  Why on earth would the journey planner advise anyone to change at Gloucester? I can't see the advantage in so doing.

Shortest-path routing engines are my speciality rather than multi-modal timetable-based ones but...

Given two different journeys producing identical outcomes, the route chosen will generally be a random, implementation-dependent pick from the two. So the trick is to weight the outcomes. WP has mentioned one way this can be done (better score for long layovers) but it should be equally possible to downrate a transfer that involves struggling over a long footbridge.

Some journey-planner software does support "preferred transfer" points, places where changing trains incurs a smaller penalty. Here's the relevant ticket for OpenTripPlanner, for example: https://github.com/opentripplanner/OpenTripPlanner/issues/365

In this case, it depends whether the software commonly used in UK^▸ (United Kingdom) rail sites supports it (I'd be surprised if it doesn't) and whether the people putting the data in have defined Cheltenham Spa as a preferred transfer point.