There are plenty of options. Weirdly not via SNCF or Trenitalia. But via B-Europe and ÖBB respectively. 

In this guide you'll find all the guidelines to reserve your seats:

,This is very difficult to answer this as there are so many routes to get reservations and then the train operators, some of whom have apps and other routes to make a change and others who don’t.

There is no single standard and, as you are finding out it is very dynamic as sites and apps change.

I am sure you will get some individual procedures for changing seats with specific operators, but even these may only work on certain platforms.

Because the Railcompanies complained that these options were often used without a real Railpass (“Fraud”) another reason is they don´t wanna include this offer anymore on their website as it was to difficult/Expensive for them. Another unspoken reason is that they hope you finally give up and buy a standard ticket were they get the entire money from you and not just a bit.

Without knowing your planned route we can just provide a general advise like @BrendanDB  already did. If you are more specific (eg. Route via witch Cities + even better the dates) we can provide detailed answers and suggestions.

Thanks for all your replies. It’s my first time doing this so I hope I’m doing it correctly.

We’ve already booked Eurostar from London to Paris. Leaving from Ashford would have been better for us but alas it’s still closed and with no sign of it ever opening again. We then go Paris to Torino, Torino to Bologna, Bologna to Rome etc eventually on to Sicily. We have Interrail 1st Class Senior 2 month rail passes. Hence our slow progress. Hope that helps. We leave on 21 March.

As I am sure you are aware, always book your reservations as one booling for 2 passengers (you only need to add one pass number id needed) then at least you should be booked together.

Once onboard you can always watch for vacant seats in first class and move - at worst you may need to return to your original seat,

Do you have a destination in Sicily? We were there earlier this year.

I’ve tried OBB for travelling by trains in Italy but, although it allowed me to book reservations, I couldn’t specify which coach or seats we wanted. Maybe it’s not possible now, but I find it very frustrating that we bought these passes in good faith expecting to be able to select whichever trains and seats we wanted — like our three previous trips with Interrail passes. Things ain’t what they used to be. It’s just that because of health reasons it would make it much easier if we could select our own seats.

OBB cant select specific seats on the Italian trains How ever the system will try to get seats next to each other :) 

Paris - torino be carefull there are the direct TGV (Interrail valid) with reservations arround 35€ per seat (these are limited booking well in advance recommended) and the direct FrecciaRossa (Interrail is not valid on FrecciaRossa in France). 

Within Italy you can catch FrecciaRossa without anyproblems just a reservation of 10€ The 1st class Interrailpass allows you to book the “Business” Class on these trains. The OBB system do it automatic when you select 1st class. 

As you will be in Italy for a long period you could leave at least some of the reservations until you there.

Any Italian station booking office will be able to do them and should be able to select the seats you want.

Unfortunately this situation where a number of the major operators refuse to fairly treat passholders by allowing them the same access to seat reservations as other customers has been allowed to happen with little apparent pushback or attempt to provide an alternative solution.

Thanks. Maybe it’s my age, but I prefer to get all of our train, hotel/airbnb and museum bookings done in advance before we leave so we have an itinerary tightly mapped out. Over the last few years (prior to Covid) our previous trips worked out well, but this year is proving to be extremely taxing.

Perhaps I should just get the Paris to Turin train on 23 March sorted out first. Is it true that the only way to reserve the TGV seats is through Interrail? Thanks again.

For domestic TGV journeys you can use the b-europe site at this link https://travel.b-europe.com/eurail-ge/en/booking-tgv#TravelWish

For international TGV the interrail reservation portal is the only online booking for these very expensive reservations.

Online, yes. But you can also call SNCF:

https://www.sncf.com/en/customer-service/contact-us/telephone

Press #85 for English, no booking fees (as opposed to the Interrail website), reservations are sent via e-mail.

I’d also have a contingency plan for travel to London if there are rail strikes. In the main Eurostar has operated to an amended timetable on strike days. 

Many thanks to you all. I’ll try phoning SNCF, but as it’s the weekend might have to wait until Monday. As for strikes, I’ll watch out for that. If it happens then we might have to go up the day before and find a cheap/ish hotel. Or pester some friends. 

Also: some railways simply do not allow ANY of their pax-not even the honored payalittleextra 1st cl- to choose seats other as window/aisle or yes/no compartmt. Just as some airls deny that.

There are also sometimes hidden motves that even seewulf will not disclose-mostly have to do with protecting staff claims.

SNCF can be reached every day:

https://www.sncf.com/en/customer-service/contact-us/telephone

It seems early afternoon is a good time to call. Don't forget to press #85 for English; you can do that immediately when they start telling the options in French.

Thanks. I phoned SNCF this afternoon, pressed 85 (my iMac doesn’t have a “hash” key) and spoke to a helpful man who spoke excellent English. Managed to select our seats with the aid of the seat plan onscreen in front of me. Cost 90 euros. Having a lie down now before thinking about the rest of our trip.

I am curious about the seat plan. I like to look out of the window. Some seats are just between the windows, so you see nothing (yet they are called windowseats!). Can you see the windows on the seat plan? Is there a website with all the seatplans (with windows!) of European trains?

Good point. The Paris to Milan seat plan doesn’t show the windows. I’d forgotten about that and, as you say, you can end up with just a bit of window. I should have asked about that. As for seat plans, the seat61.com website is really helpful — about lots of things. Go to the Train seat numbering plans page.

The dreaded "Wandfensterplätze". Brrrr. They should offer those seats with a discount, honestly. 

Deutsche bahn usually has a proper seating plan of their trains if you dig a bit deeper on the website. Or if you google something like "sitzplan ICE 3" or something. But then you already need to know which kind a train is running on your route.

For the UK this is usually found easily as well, often via the man in seat 61 though.

But for the rest of Europe, no idea. Curious if someone can enlighten us about this. 

I use the website of vagonweb.cz and use the pictures to identify my prefered seats :D

In Case of Italy i recommend to reserve guranteed trains Italy is famous for strikes But Trenitalia have a list of guranteed trains that even runs on strike days https://www.trenitalia.com/it/informazioni/treni_garantiti_incasodisciopero.html

In Sweden you can select your seat according to the direction of the train and the location of the windows.

LNER in UK allows you to change your seat online for no cost (Lon East Coast line Lon - York, Leeds, Darlington, Newcastle, Edinburgh and Aberdeen).

https://www.lner.co.uk/travel-information/make-a-reservation/

Indeed. I call them "French windows”, similar to a "French balcony”, which isn't a real balcony...

Seewolf wrote:

In Case of Italy i recommend to reserve guranteed trains  Italy is famous for strikes  But Trenitalia have a list of guranteed trains that even runs on strike days

Thanks. How do I find this list of guaranteed trains, it sounds really useful.