What marks the end of a London zone journey on an add-on to a rail ticket?

[mm333]

The rules say it’s valid for one journey, on, say a ZONE U123* LONDN add-on.

I’m coming to London and staying at a hotel in Canning Town. In order to ride some of the Elizabeth line - and because I’ve never bought an add-on ticket before, I made Custom House EL my destination and TrainSplit suggested a single to, what they call, “London Underground Zones 1-3” and BR Fares calls “ZONE U123* LONDN”.

So if my route, having got into Kings Cross, is Thameslink from St Pancras-Farringdon, Elizabeth line to Custom House and then DLR to Canning Town, could I use the add-on to get me all the way on that route to Canning Town?

When does the one journey add-on end? Is it when makes a change that isn’t part of an OSI or you try and go back through a gateline you’ve already left?

 

[JBuchananGB]

The TfL Journey Planner produces that route as a valid option when asked to generate a route without using the Tube, so seems to be no reason to doubt that it is OK.

 

[pelli]

At Custom House, the Elizabeth Line is behind barriers but the DLR is outside, so you'll need to exit the gateline to go from EL to DLR. I'm not a fares expert, but I note that the Out-of-Station Interchange information page on the TFL website states

Across our Underground and Rail network we offer a similar but more limited arrangement for customers with paper tickets

meaning that OSIs for Pay As You Go on Oyster/contactless can be different from OSIs for paper tickets. Indeed, the Excel file linked to by that page shows most as being "PAYG only" whereas a much smaller number are listed as "Magnetic and PAYG" (where I presume "Magnetic" means paper tickets): various connections with Marylebone, Bank, Euston, King's Cross St Pancras, Liverpool St, Fenchurch St, Paddington, Tower Hill, Canary Wharf, as well as the pairs Clapham North-Clapham High St and Wood Lane-White City.

Notably, Custom House DLR to/from Custom House EL is listed as "PAYG only". It's not clear whether this omission is deliberate, because by contrast, some other connections between barriered stations and unbarriered DLR stations are shown as "Magnetic and PAYG", such as Tower Hill to Tower Gateway DLR and Canary Wharf LU to Heron Quays DLR, but other equally sensible ones such as Canary Wharf LU to Canary Wharf DLR (which the system couldn't possibly distinguish from Heron Quays DLR since you don't validate your paper ticket at the DLR station) are "PAYG only".

If there is no "Magnetic" OSI set at Custom House, then presumably your main obstacle would be exiting the barriers without getting your ticket swallowed (if the barriers can do that?) or otherwise marked as not valid for further travel, because even though there are no barriers to enter for the DLR you will need the ticket to exit the barriers at Canning Town (and also to show it if inspected on the DLR of course).

Edit: Canary Wharf EL to LU is listed as "Magnetic and PAYG" so at the very least it should be legit to take the Elizabeth Line to Canary Wharf and change for the Jubilee Line from there to Canning Town. Another valid option is Canary Wharf EL to Heron Quays DLR, from which you can take the DLR to Poplar and change for another DLR to Canning Town. Unfortunately Canary Wharf EL to Poplar DLR, West India Quay DLR, and Canary Wharf DLR which are all much less of a walk than to Heron Quays DLR are listed as "PAYG only", so strictly speaking you're not allowed to pick up the DLR directly from those stations, but I'm sure TfL won't mind! In case you're unfamiliar with the Canary Wharf area, here's a map with the stations and interchange routes labelled:

 

[mm333]

Thanks @pelli for your informative and interesting post.

To feed back what happened, my ticket was accepted and not kept by the barriers at both Custom House EL and Canning Town.