First of all, you need to count how many pass days you’ll likely need. On a pass day (00:00 - 23:59) you can board as many trains as you wish.

You can’t change the duration of the pass so better take a slightly longer one and buy separate tickets if needed. I guess the 15 days in 2 months pass should be good enough ?

Public transport within cities is not covered. The only exception would be S-Bahn (suburban trains) in Germany or Austria but it’s not good value to use a pass day just for these.

Seat reservations are mandatory on fast/high-speed trains depending on the country. Some routes are more popular than others and need booking well in advance, those are also the most expensive… thinking of :

• TGV Barcelona - Paris 35€
• Eurostar to/from London 30€ (passholder quota too)
• “continental” Eurostar (ex-Thalys) 27-32€ (passholder quota)

It is often (always) cheaper to book them elsewhere than on eurail.com as they add 2€ fee per person per train, it quickly adds up!

Feel free to read these 2 links :

https://www.seat61.com/how-to-use-a-eurail-pass.htm

https://www.seat61.com/interrail-and-eurail-reservations.htm

There are ways to save money (for example taking a regional train across the Spanish border and then a TGV high-speed train to Paris) but with limited time you might prefer to take the most direct options, even if they cost some money.

When is your trip planned ? I’d recommend booking mandatory reservations well in advance, especially for travel this summer or around holiday weekends. Paris is hosting the Olympics this summer as well so expect very high prices, more crowds than usual and extra security measures (honestly I’d avoid the city around that time).

Questions welcome. :)

The Eurail pass covers most trains in Europe, about 97% percent of all trains. The pass works by companies. If a company participates, you can take almost all trains of them (few exceptions). Only a few private operators, and some touristic mountain railways are not included.

Your intercity travel will be covered. (Intercity or IC is also a train category you’ll come across a lot in your travels ).

Intracity travel or city transport (city bus, tram, metro, Parisian RER) are not covered. Suburban rail however (like London Overground, London Elizabeth Line, German S-bahn,...) are covered. But usually not worth a pass day, as tickets are very cheap (only a few Euro’s usually, even cheaper for young kids).

So, which pass to choose?

You should count not your travels as such, but your travel days (all the days on which you could potentially take a train). In your case I would doubt between a 15 day flexi pass (usable in two months) or a monthly continuous pass. With this flexi 15 day pass you might run a bit tight on travel days.

Personally, I’lld go for a one or two months continuous pass. The price difference between a one and two months pass is actually not that big, for 120 EUR more you get a complete month extra. Or buy a month pass, and buy your last tickets as ordinary tickets if they fall outside that month, but I think that’s gonna be more expensive than taking a two months pass. But make sure to compare well between ordinary tickets for your last 3 days of travel, vs. the surcost of a two monthly pass.

The good side of a continuous pass is that you get a lot of extra travel days. If you decide to do a day trip to smaller cities (e.g. a day trip from Brussels to Bruges or Ghent), or you want to get away from the busy big cities and go for a day trip in nature, the continuous monthly passes give you a lot more flexibility for spontaneous and impulsive decisions to visit something.

Other recommendation:

Spain and France are rather tricky with a rail pass, as they have strict reservation policies that limit flexibility. If you’re still able to replan a bit, I’lld start in an easy interrail country, that only has optional reservations (The UK, Belgium, Netherlands, Luxemburg, Germany, Austria, Switzerland,...) once you get the hang of that, you’ll have no issue railing around France and Spain.

I see @thibcabe already said something about reservations ;)

I’lld suggest you do some further reading too:

https://www.seat61.com/how-to-use-a-eurail-pass.htm

https://www.seat61.com/european-train-travel.htm

https://www.seat61.com/interrail-and-eurail-reservations.htm

Feel free to ask any other questions if something is unclear.