The coverage works by train companies and the vast majority of European trains are included. High-speed trains definitely are while suburban trains aren't always. Subways and trams are never included.

Notable exceptions where Eurail isn't valid :

- RER network in Paris

- Ouigo low-cost high-speed trains in France. There are always normal TGVs on the same route.

- .Italo in Italy, Ouigo, Iryo, Avlo in Spain

Eurail is fully valid on boats on Lake Thun and Brienz (Switzerland), you also get discounts on multiple lakes and mountain railways there too.

Please note that you need seat reservations on fast/high-speed trains depending on the country/company (extra costs). Some popular routes need to be booked well in advance.

Have a look at this general guide : https://www.seat61.com/interrail-and-eurail-reservations.htm

Questions welcome.