European rail passes
To save money on a multiple destination trip you may want to look into purchasing InterRail, Eurail or some of regional passes. "Interrailing" is less popular in these days of discount airlines and various affordable air passes, but it remains a uniquely flexible way to travel — you can literally arrive at a city, decide you don't like the look of it, and zoom off on the next train out. This makes it a great way to get a feel for a large region, especially when heading out into the countryside. Do not, however, fall into the trap of traveling so continuously that all you see is a blur of railway stations.
Rail passes work just like tickets. After validating the pass, the pass holder is free to board any train that does not require reservations and is within the area/countries specified on the pass.
In general, passes are valid only on trains operated by national rail companies. In some countries (Italy, Spain and Switzerland in particular) you'll find regional or private companies that don't accept InterRail or Eurail passes, although many of them offer up to 50% discount for passholders.
Extra fees can apply for making reservations, fast trains, couchettes and sleepers. The exact rules vary by country and can be very complex, so ask in advance, but a rule of thumb is that anything which requires a reservation in advance (shown with a black [R] in a box in schedules) will require a surcharge. In peak season on popular routes seat reservations are definitely worthwhile.
If travelling overnight, the token fees for couchettes (usually less than €20) are well worth the price.
High-speed trains such as TGV, Thalys, ICE (only lines to France), Eurostar Italia, Cisalpino, X2000, AVE and Talgo 200, may require pass holders to pay supplements. Passes are not valid on Eurostar  crossings between UK and France or Belgium, but a discounted Passholder Fare applies to those with valid railpasses for travel in France or Belgium (departure or arrival country).
The complete list of trains with compulsory reservation you can find at InterRail website .
On both InterRail and Eurail, ferries between Ireland and France, Italy and Greece as well as many ferries in the Baltic sea between Denmark, Germany, Sweden and Finland, are either free or steeply discounted. Many boat rides on Switzerland's lakes are free as well.
The exact conditions vary according to the ferry operator.
Always check the daily schedules during the specific week of travel. Some ferries cease operation in the off-season altogether, while others reduce service to one roundtrip daily, from several trips a day during peak season.
Travel days are generally counted from midnight to midnight. There is one useful exception: If you board a direct overnight train or ferry after 7PM, your travel day will last until midnight the next day.
Quirks and caveats
Unless otherwise noted, these all apply to both Eurail and InterRail passes.
Eurail Passes cover border city stations outside their countries as well. For example, Salzburg in Austria is considered a border station of Germany and therefore is covered by German railpasses.
A vacant seat is not guaranteed unless you make a reservation.
One-month passes last longer when validated (on any day) within a 31-day month.
Passengers with 1st class passes may travel in second-class compartments at any time. Those with 2nd class passes can pay the difference (generally 50%) between 1st and 2nd point-to-point fares to upgrade to 1st.
Check the actual prices of normal point-to-point tickets, in some case your journey can be cheaper with them than with a pass. Especially in eastern European countries a pass tends to be bad value for money as the local cost of point-to-point tickets is very low.
The InterRail  pass allows any person who has been a legal resident in Europe or any of Russia, Belarus, Ukraine, Moldova, Algeria, Morocco or Tunisia for at least six consecutive months (not travelling on a visa, or military personnel living on a base), to travel throughout Europe by train.
Types and prices
The previous convoluted zone system has been abolished, and there are now only two basic types:
Every type has a variant for travelling in 1st and 2nd class. There are discounted passes:
Children under 4 years can travel free.
The price for Global range from €169 for 5-in-10 2nd class Youth pass to €929 for month 1nd class adult pass. The one country pass prices vary more from country to country.
Rules of use
The actual pass is a booklet the size of an airline ticket, each page filled with rows and columns. The front page will state the validity of the ticket (zones and time) and your personal details, which must match the ID you are using (usually a passport). Using it is very easy: whenever you board a train, write down date and time, where you're going from, where you're going to, seat or couchette, and the train number. When the conductors come to check tickets, show them the pass and they'll (usually) stamp that row. That's it! If you manage to run out of pages — a sign that you're travelling way too much — you can get extra ones added on at any larger train station. Your InterRail pass cannot be refunded if lost or stolen, so guard it carefully!
Also note the one big exception of InterRail: travel in your home country is not included. Most countries do, however, grant a 50% discount for the trip to the nearest border. The same discount also applies if traveling from zone to zone through a country outside the pass.
Eurail  is a variety of rail passes which cover travel in a total of 22 countries in Europe: Austria (including Liechtenstein), Belgium, Bulgaria, Croatia, Czechia, Denmark, Finland, France (including Monaco), Germany, Greece, Hungary, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Romania, Slovenia, Spain, Sweden and Switzerland.
Intended for foreign visitors to Europe, the pass is similar in scope to Inter Rail, which is exclusively for European residents. Eurail Passes and Eurail tickets may not be sold to residents of European countries, Turkey, Russia, Morocco, Algeria and Tunisia.
Eurail Global Pass
The Global Pass covers unlimited travel in all 22 countries. They are available in fixed-length versions of 15 or 21 days, and 1, 2, or 3 months of consecutive-day travel, or Flexipass, that allows the passenger to choose 10 or 15 non-consecutive days of travel within a period of 2 months.
Eurail Select pass
The Select pass allows you unlimited travel in 3, 4 or 5 adjoining countries for 5, 6, 8, 10, or 15 travel days in a 2-month period (the 15-days version is only available for 5 countries).
Eurail Regional Pass
For 3-10 days of travel in a 2 month period between two bordering countries connected by train or ship.
Available combinations are: Austria-Croatia/Slovenia, Austria-Czechia, Austria-Germany, Austria-Hungary, Austria-Switzerland, Benelux-France, Benelux-Germany, Croatia/Slovenia-Hungary, Denmark-Germany, France-Germany, France-Italy, France-Spain, France-Switzerland, Germany-Switzerland, Greece-Italy, Hungary-Romania, Italy-Spain, Portugal-Spain.
Eurail National Pass
National passes are valid in one of the following countries: Benelux, Croatia, Denmark, Finland, Greece, Hungary, Ireland, Italy, Netherlands, Norway, Poland, Portugal, Romania, Spain, Sweden.
The following are counted as one country for the Select Passes and National Passes:
Pricing naturally depends on the exact variation: a flexible 5-day 3-country Youth Select Pass starts at $265, a consecutive 15-day Global Youth pass can be yours for $415, while a "travel as much as you can" consecutive three-month 1st-class pass would set you back a whopping $1789.
There are no Senior rates for Global or Select passes, but Senior rail passes are sold for specific countries or regions -- France, the Balkans and Scandinavia. These passes are available in 1st-class only, and cost little more than 2nd-class passes.
Prices usually rise every new year to reflect the changes in exchange rate and point-to-point fares, but as passes are generally valid for six months from date of issue to first day of travel, if you got your travel plans fixed it would make sense to buy passes in December, yet travel as late as June of the following year.
Passes are 85% refundable if cancelled before being validated, but after validation no refund is available for unused days of travel. Customers are offered an optional pass protection, which allows refund of point-to-point tickets bought within the scope of the pass in case the pass is lost or stolen.
Children under 4 travel free, except if a reservation for a separate seat is requested. Children 4-11 receive 50% off any Adult pass when accompanied by an adult with the same pass.
Rules of use
Passes must be validated by a railway agent prior to first day of use. Holders of non-consecutive days passes should mark the date in the appropriate box before boarding a train or ship for the first time each day.
Unlike Inter Rail, there are no limitations regarding the starting country, and there are no discounts for travel outside the selected zones.
Although the slow and infrequent trains are by no means the most efficient way of traveling in the Balkans (this is by any standard of the bus), it is one of the more comfortable and scenic.
The Balkan Flexipass allows unlimited rail travel for 5, 10, 15 days of rail travel in a 1 month period in Bulgaria, Greece, Macedonia, Montenegro, Romania, Serbia and Turkey (including the Asian part). You can buy the Balkan Flexipass at train stations in the countries mentioned  or, much more expensive, on the web.
Caveat is that trains in the Balkans are already really cheap and that it only pays off for longer distances.
Some countries, for example Britain, Czechia, Germany and Switzerland, offer they own one-country pass. See train travel section of the concrete country for more information.