It’s usually high-speed and/or reservation-mandatory trains that you have to pay supplement for. But it varies from one place to another. What countries are you going to?

norway, sweden, denmark, germany, italy, france, spain, amsterdam, brussels..are the countries i am hoping to travel to!!!

This is what the[url=‘http://www.b-rail.be/int/E/offers/interrail/price/index.html’]Belgian railways website[/url] says:

quote:
Supplements


In some cases, however, you will have to pay a supplement: to travel first class (except in the Asian part of Turkey);
for nighttime comfort (sleepers);
to use certain EuroCity or high speed trains such as:
– TALGO, AVE and Euromed in Spain;
– EC/IC in Germany, Denmark, Finland, Greece, Hungary, Italy, the Netherlands, Poland, Portugal, Slovakia, the Czech Republic and Romania;
– TGV in France;
– ICE and ICE-Sprinter in Germany.

Holders of Inter Rail passes valid for zone E can travel by Thalys, Eurostar and TGV by purchasing a ticket at the &quotass" rate.
Holders of Inter Rail passes valid for zone A can travel with Eurostar by buying a ticket at the &quotass" rate.

Find out in your local station before boarding the train!

When you buy your pass, you’ll probably get a brochure that explains on which trains it’s valid and when you have to pay a supplement, and boats you get a reduction for etc. On IC trains, supplements are usually fairly reasonable, but try to avoid TGV, ICE or other high speed trains.

PS: I hope you realize that Amsterdam and Brussels are not countries.

"Signature" trains in Norway, and "Lynx" trains in Sweden. Lynx trains go as far as Copenhagen and Oslo in cooperation with Danish and Norwegian railways. See www.nsb.no www.dsb.dk www.sj.se
Supplement about 50kr for these if I recall correctly. Well worth it as these are some of the best(?) trains in the world—modern, clean, comfortable. 2nd class is very good. Nothing like the crappy IC train you change to at the German border (Padborg?). Stinky soot-cover gulag boxes with 60s-era vinyl bench seats facing 3-3 in compartments, you can hear peeing and wooing at the same time next door and track splatters—and that’s at the station. Night and day difference compared with the luxurious, modern Danish trains north of the border.

For the TGV in France, reservations are required, and cost 4 euro.

Sorry I can’t post this as a direct link:
http://railpass.com/…
Click on "info center" then "using your pass" then "supplements" for a complete list, by country, of the trains in the 17 country eurail system that require supplements or reservations. Price given is approximate price if you buy reservation or pay supplement in Europe.