Interesting. Great Britain does not accept the Eurail pass. Neither does the Eurostar although you can get a passholder fare that can be purchased at Eurostar terminals. Note that you have to be checked in 30 minutes early on the Eurostar.
Eurail passholders will pay supplemental or reservations fees on several types of trains in Europe. One of the worst is the Thalys train from Paris to Amsterdam. 39Euros for 2nd class and 62Euros for 1st Class. Odd that you can buy a non-refundable ticket on the Thalys train in advance with seat reservations online from Paris to Amsterdam for 35Euro which is less than the supplemental fee. The supplemental fees include the seat reservations. Note also that Thalys limits their seats to passholders. If no seats available on a certain train run, you either pay full fare for that run or wait for later trains where passholder seats are available.
For Germany and most of Switzerland, you are fine on most of their trains. Just the ICE trains have reservation fees. The ICE IC and EC trains have a 4Euro reservation fee. The ICE Sprinter has an 11.50 fee for 2nd class and a 16.50 fee for 1st class. For Switzerland, your Eurail pass will only cover you on 25% of the transportation fares (train, gondola, boat) from Interlaken Ost into the Lauterbrunnen Valley. This includes Lauterbrunnen, Wengen, Muerren, Gimmelwald and travel to/from the two mountain tops (Jungfrau & Schilthorn). You pay 75% of the transportation fare. The bus inside the valley does not accept the pass. All four Swiss scenic train runs have extra fees except the Golden Pass. The Glacier Express has a 27Euro fee in summer. The Wilhelm Tell Express has a 70Euro fee for 2nd class. The Bernina Express has a 10Euro fee in summer. The Chocolate Train has a 46Euro fee for 2nd class.
