Reservation and itinerary from Amsterdam to Naples

Hi Bob

Best plan your trip via Deutsche Bahn, it has up to date schedules and itineraries for most of Europe. Give your search request here: https://www.bahn.com/nl. You can also reserve your seats on German trains their (select "seat reservations only" when you plan).

I think you want to split your journey in two parts? I would advice you travelling over Switzerland. Most trains will have only optional seat reservations then. And you avoid some very expensive seat reservations (like on Thalys) and a cumbersome switch of trains and stations in Paris. 

For the trains to and in Italy, you'll need seat reservations. 

You can get these the easiest via ÖBB (Austrian Railways): https://www.oebb.at/en/. Plan a stretch of your trip (e.g. MILAN- NAPOLI), add a discount in the travellers information (Interrail /Eurail) continue, and you'll see seat reservations for about €10 per seat, per train for high speed trains; €3 for IC's. 

I think there's also a supplement on 1 cross border EC  train from Switzerland to Italy, but others can advice you better on that.