Anyone who has backpacked around Europe likely is familiar with the concept of the Eurail pass: a train pass designed for those who like to keep their itineraries wide open, allowing rail travel throughout the continent. If you wake up in Venice and say, "I think I'll go to Berlin today and maybe Barcelona tomorrow," the Eurail pass gives you that flexibility without having to buy a ticket for those routes. You'll also avoid standing in ticket lines at European train stations (which can be long) or having to order online.

The passes must be purchased in advance, before arriving in Europe. They come in many varieties: Global Pass (up to 28 countries), Select Pass (four bordering countries), Regional Pass (two bordering countries), and the self-explanatory One Country Pass. They can be used for a range of travel days (such as 15 continuous days or 10 days in a month). Pricing is determined by type of pass and number of travel days.

Flexibility comes at a price. For example, a pass for 10 days of unlimited rail travel within France, Germany, Italy, and Spain during a two-month stay costs $684. So it's important to determine what type of trip you'll be taking. If journeying to only a few destinations on a fixed itinerary, you might be better off buying individual tickets.

It's important to note that for travelers 26 and older, the Eurail pass is available only in first class - unfortunate, as European trains are quite comfortable in (less costly) second class. There may also be some added fees. During our recent trip to Europe, we learned high-speed trains in Italy required a seat reservation fee on top of our prepaid pass.

A good resource for comparison and planning is RailEurope (www.RailEurope.com). It offers the ability to price train tickets for individual routes you might take on your European journey, along with pricing for the various Eurail passes. Factor in the freedom and flexibility that Eurail provides to make your decision.

For more information about Eurail passes, go to www.Eurail.com.

Philadelphia natives Larissa and Michael Milne have been traveling the world full-time since 2011. Get more travel tips on their blog, www.ChangesInLongitude.com.