No it is what  it is and its also clearly stated on the country-info. There is 1 other train comp in IT that also runs the hi-speed but it does NOT accept passes-its private/commercial. reason-with just a tiny bit of thinking: prevent fraud by people who reserve and do not have/buy the ticket.

Most worries are completely unfounded that trains are f´full., weeks ahead-just do it at a counter when you have arrived in IT-does not need to be dep. stazione

You can book Trenitalia day trains via ÖBB using these steps.

I tried with ÖBB but now is not working, there's no seat shown. If you know other website can buy seat, let me know. 

Yeah, it doesn’t give an option to buy a ticket. 

It shows seats to me:

If you don't see prices, then you didn't follow the steps. You have to add the discount "Interrail / Eurail”. If you did follow the the steps, then please post a screenshot where you are stuck.

It shows seats to me:

If you don't see prices, then you didn't follow the steps. You have to add the discount "Interrail / Eurail”. If you did follow the the steps, then please post a screenshot where you are stuck.

I already do the steps and still didn't show the price. 

It shows seats to me:

If you don't see prices, then you didn't follow the steps. You have to add the discount "Interrail / Eurail”. If you did follow the the steps, then please post a screenshot where you are stuck.

I already do the steps and still didn't show the price. 

Mine looks the same as this. I put “add discount” and then “Interrail Global pass” and the same come up and the above 

I still see no problem. Tested with an 11 year old child and  2 adults:

Questions:

1. Did you maybe select "Seat only”? If so, please follow the steps and select "One-way tickets and day tickets”.
2. Did you also enter the "Eurail / Interrail” discount for the child?

I still see no problem. Tested with an 11 year old child and  2 adults:

Questions:

1. Did you maybe select "Seat only”? If so, please follow the steps and select "One-way tickets and day tickets”.
2. Did you also enter the "Eurail / Interrail” discount for the child?

So I used the “One way Tickets” option and it worked for 2 adults and the Interrail pass discount! €20 for everyone. Thank you for the help! 

I still see no problem. Tested with an 11 year old child and  2 adults:

Questions:

1. Did you maybe select "Seat only”? If so, please follow the steps and select "One-way tickets and day tickets”.
2. Did you also enter the "Eurail / Interrail” discount for the child?

So I used the “One way Tickets” option and it worked for 2 adults and the Interrail pass discount! €20 for everyone. Thank you for the help! 

I just know about buy seat with add disc 'interrail/eurail global pass'.

Before i bought seat from germany-italy in öbb, direct to seat only without add disc interrail/eurail global pass and the price can come out.. i'll already success bought the ticket, my question is it become problem when in train the ticket inspectour check my seat because i didn't connect to my interrail pass.. ?

The EC Germany - Italy has optional seat reservations, which means you can buy it from ÖBB with the "Seat only” option. The rail planner app says reservation is mandatory but that is incorrect. Since reservation is optional, nobody will ask for it. There is however a pass supplement for these trains (10€ in 2nd, 15€ in 1st class), but you can buy that from the ticket inspector for the same price, if they ask for it (some don't).

I see.. if the price didn't come out after i add disc  interrail pass, that's mean there is no seat or full.. so if i force to take the journey, is it okay ?? as you say that seat is optional.

Should be OK. What is you travel date and departure time and from which station?

Firenze city - würzburg, 17 apr, 22.31 - 11.52(+1)

Right, so we see is is important always to specify exactly which train you want to use. This is the night train Firenze - Munich, which does have mandatory reservations, and which seems fully booked for the 17 April departure. Which is not strange, given that it's Easter weekend. There are still seats on 18, 19 and 20 April. First date with couchettes only on 21 April.

If you want to travel on the evening of the 17th, the night trains Venice - München (change in Venezia Mestre) and Milan - München (change in Padova) still have seats left (no couchette or sleeper):

For München - Würzburg, I wouldn't bother with a reservation.

Set "via Padova” to get these results.

I see.. if the price didn't come out after i add disc  interrail pass, that's mean there is no seat or full.. so if i force to take the journey, is it okay ?? as you say that seat is optional.

Should be OK. What is you travel date and departure time and from which station?

Firenze city - würzburg, 17 apr, 22.31 - 11.52(+1)

Right, so we see is is important always to specify exactly which train you want to use. This is the night train Firenze - Munich, which does have mandatory reservations, and which seems fully booked for the 17 April departure. Which is not strange, given that it's Easter weekend. There are still seats on 18, 19 and 20 April. First date with couchettes only on 21 April.

If you want to travel on the evening of the 17th, the night trains Venice - München (change in Venezia Mestre) and Milan - München (change in Padova) still have seats left (no couchette or sleeper):

For München - Würzburg, I wouldn't bother with a reservation.

Set "via Padova” to get these results.

Ok, thank you very much..

I still have question.. after my journey in/outbound completely use in global pass. actually i still have 2 days left for trip.. can i use it in other date within 1 month in germany ?? *My residence in germany. 

You can travel in your country of residence on 2 of your travel days. After you have used both of these, you cannot use your Interrail anymore in your country of residence.