You can see that on trenitalia.com. Start the booking process for your train and click on "Choose seats”.

Thank you! 
 

on one of my trips, ive tried to do as suggested, but my coach doesn’t exist? Will it be added later on? 

Are you looking in the right class? Italian trains can have up to 4 classes.

Yes, on this train there’s only 3 coaches. Quite sure they’re all the same class, but there’s only coach 1 , 2 and 3. Im meant to be in coach 5? My other two trains are fine using the method you said, but there’s just this one where I can’t find the coach!!

On second look, there’s only 1st class seats available in all 3 coaches available. Where are the 2nd class coaches!! It’s Milano Centrale - Livorno Centrale 6th June 2024, train FB8619 at 1:10pm

Coach 5 in FB8619 is 2nd class.

Where did you book? Maybe the reservation can be changed.

I haven’t found a definitive seating plan but your train has a full complement of 9 carriages. That service is operated using the ETR 460 train set (an early tilting train). It seems many have been renumbered and similar train seating maps show 8A and 9A as window seats opposite each other (B is the aisle) with a table.

Booked a seat ticket through the interail website. On the trenitalia website I can only find it with 3 carriages, not sure where everyone else is finding carriage number 5 but at least it’s there somewhere that’s reassuring 

If you select a second class ticket on trenitalia.com (16.90€) you'll then see a seat map of carriages 5-9 -> all second class.

If you booked your seat reservation through Interrail, then it's possibly a booking error. There have been similar issues in the past, where Interrail booked a different category than requested. Are you sure you selected 1st class when you made your reservation?

In any case, the reservation changes are unlimited and free of charge. It should be possible from the Reservations overview in your Interrail account. I'm not sure if changing from 2nd to 1st class is possible, but you never know.

If that doesn't work, then please contact customer service:

https://eurail.zendesk.com/hc/en-001/requests/new

For any future reservations in Italy: Raileurope.com and tickets.oebb.at are cheaper (no booking fees).

Thank you everyone! I’ve now found coach 5, realising I was booking in first class by mistake. 

Also, thank you for the advice on raileurope.com, will use that next time :)