Traveling in Europe (train or rental car?)

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I'm trying to make a decision on the means of transportation to travel around Europe. There are 4 of us. At first, train (Eurail Pass) seems to be the best option, but comparing it with renting a middle sized car (+ you have to buy the gas, pay for the parking, etc) I come up with about the same amount of money (if not less..). Any experiences? Inputs?