Especially if travelling using InterRail tickets.
Needed if travelling with InterRail tickets
LNER
https://www.lner.co.uk/reservation-engine No
fee
TGV Lyria
In person at any SBB travel office - I think this is free https://www.b-europe.com/EN/Booking/Pass Costs €4.00 per
reservation - covers multiple trains & multiple persons
https://www.interrail.eu/en/book-reservations/ Costs €2
per passenger per train - cheaper for 1 person & train, otherwise
use b-europe
EuroStar
https://www.b-europe.com/EN/Booking/Pass Costs €4.00 per
reservation - covers multiple trains & multiple persons
https://www.interrail.eu/en/book-reservations/ Costs €2
per passenger per train - cheaper for 1 person & train, otherwise
use b-europe
LNER will send an email and you can use that to change your seat
For Eurostar I add the reservation to the Eurostar app and use that for seat selection but the website is also good
TGV Lyria - tricky, last time we needed to do this we went to the
office at Gare de Lyon and used the website there.
There should be a way using https://www.sncf-connect.com/ but I’m not sure.