See:

1. The Night train Edinburgh London only counts as 1 travel day, but since, you use the Eurostar the 2nd day, that would use a 2nd (and you will have used your 2 inbound/outboud travel days).
2. No, the possibility to book in 1st class when using a 2nd class pass is very rare and you cannot do it on these trains

I suggest to take the Caledonian Sleeper, which departs from Edinburgh at 23:40 local time (00:40 Central European Time). Travel days count from midnight CET, so you will spend only 1 travel day. You could arrive in Paris at 11:20.

Except that the app doesn't adhere to the conditions and uses local time. You need to either ask for an extra travel day and inbound/outbound to customer service or enter the train manually with a departure time after midnight.

Hi , I am exploring this option, but Eurail/Interail seat reservation showing change in train at 00:11 at Carstairs. Is this a bug ?

but when i checked Caledonian website Buy train tickets online | Caledonian Sleeper train tickets it not showing any change.  Also here i don't see any option to add interrail/eurail pass information. How can i reserve a seat for this train?

TIA.

That's a data error somewhere. I've seen this repeatedly in the past. Just enter this train manually - you need to do that anyway as long as the app doesn't adhere to the conditions of use.

@Mukhammad @Camilo. Can this be fixed in the app? The DB planner doesn't show a change so that could mean the MERITS data is correct and the problem is in the data the app gets.

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Thanks for reply @rvdborgt . Few more please :)

1. If you could also suggest how to reserve seat (sitting only) in this train as i don't see any option to add interrail/Eurail pass in Caledonian website. its showing GBP 50. Is this price for reservation only or reservation with ticket?
2. Also, if i will take this train (Caledonian Sleeper, which departs from Edinburgh at 23:40 local time) what is the possibility i can catch 9.31 Eurostar from London St. Pancras for Paris considering distance from London Euston to London Pancras, time for security check for border and bag and door closing 30 mins before? 
3. what date and time to put in app if i want to add train manually - 17/05 23.40 local time or 18/05 00.40 CET?

Sorry for asking too many questions as I have no prior experience with Europe rails and interrail/Eurail passes., so kindly bear me :)

1. Check the box "Room supplement only”. However, I see it then says:

   

   It might also work to book using GWR. In any case, I do see the train there.

2. Euston is just a 10-minute walk from St. Pancras. The ideal time to join the queue for the Eurostar check-in is 1 hour before departure time, so you should have time left for breakfast somewhere close.

3. Use 18/5 with a time after midnight, indeed e.g. 0:40.

@Mukhammad @Camilo. Any news on from when the app will adhere to the definition of a travel day?

Carstairs is the junction where the Edinburgh portion of the train and the Glasgow portion join together (or split up, when going northbound) That’s possibly why it appears as two trains in the data, but it’s definitely a direct train.

I agree with @rvdborgt that the 0931 is a fine connection. It’s a ten minute walk, and you’ll be at St Pancras with time for a leisurely breakfast.

There’s no bag drop. Just turn up at the ticket gates about an hour before departure. You will go through ticket check, then security, then British passport control, then French. It’s all very smooth, one multi-step process, and then you’re in the departure lounge waiting for them to announce boarding.

@rvdborgt Thank you for pointing this out. We have reported to issue. This is a long weekend so the earliest it might be addressed is Tuesday. 

I tried both https://www.sleeper.scot/  (with Room Supplement Only checked) and https://myaccount.gwr.com/ for various random dates from today till Sep, but it always returning grey for Caledonian website and “Unable to make reservations. Seat not available” for GWR. It's weird for none of dates from today until sep  i have found the seats. Am i doing something wrong?

If it doesn't work, then do contact Caledonian Sleeper's customer service, as they suggest.

Hi,

I wrote to customer care, but they are not understanding the issue that app is not adhering to the conditions and using local time. I have also gave example of the journey I am interested in of Caledonian sleeper which run from Edinburgh at 17/05 at 23.40. can I loop someone from eurail team who is aware of this issue.