The length doesn't matter but the type of train does. Frecciarossa (high-speed) is always 13€.

Advance tickets in Italy are cheap (often cheaper than a pass day + reservation) but at the last minute a pass becomes useful -> fixed reservation price while tickets triple.

Availability in Italy is not an issue a few days in advance, you don't have to book right now if you prefer to remain flexible.

Book elsewhere than on interrail.eu as the website adds 2€ per person per train. See: https://www.seat61.com/interrail-and-eurail-reservations.htm

What's your journey? There might be slightly slower reservation-free regional trains along the same route. Regional train timetables might not be published yet for the summer.

sorry for the delay in responding and thank you for your response. We have a global flexi pass in 1st class. Our plan is to travel from Rome Termini to Florence, then on to Salerno, next will be Tropea and finally back to Rome.

I did try book seat reservations on Austrian OBB but every trip said no tickets available. 

One more question.  Can I book Leonardo Express and then  book seat reservations at the airport ?

Can’t seem to make a reservations online.

For OBB you must add Interrail/Eurail as a discount and then select one-way tickets. That way the passholder fare will be shown. Otherwise feel free to use https://www.raileurope.com/ : desktop version only, click on add rail pass (no fees and 2€ extra for seat selection).

Btw if those are your only journeys for the whole trip a pass will not be good value.

The Leonardo is a suburban/regional train and seat reservations are not possible. Simply board the first train you can. (It is classified as 1st class only as all airport trains are expensive across the world but in fact it’s a standard regional train.)