Overall, there are more than 23000 km (about 14300 miles) of conventional tracks and about 1350 km (840 miles) exclusively assigned to high-speed transportation in Italy. This extensive network is mainly due to the presence of a big national rail service, Ferrovie dello Stato (also known as Trenitalia) which connects basically all the Italian regions. Besides Trenitalia, Italy is served by other local or private rail companies, like NTV Italo Treno, the most famous one.
Italo Treno is dedicated to the high-speed train connections between the biggest Italian cities, like Rome, Milan, Naples and Venice. Trenitalia has both a high-speed offer, with the Frecciarossa, Frecciabianca and Frecciargento trains, and a more local and cheaper offer, with the regional trains that connect small towns and Intercity, which cover long distance national journeys.
There are also other train companies which operate mainly at regional level, like Trenord, which connects Milan to nearby destinations, like for instance the Como Lake and Maggiore Lake areas, or Ferrovie del Sud-Est, which connects all the towns in Apulia, getting to famous seaside destinations like Gallipoli or Bari.
Train tickets in Italy are actually rather cheap if compared to other European countries, especially when booked a few weeks in advance: in the last years, more and more people choose high-speed trains to travel from one Italian city to another, and rail companies like Trenitalia and Italo keep offering special fares and promotions. In other words, you can actually speak of low-cost trains. An example? A Frecciarossa (high-speed train) from Naples to Rome can cost as low as 19€ (that is to say 20$ or 16£) and you’ll get to your destination in just one hour enjoying maximum comfort.