Totally depends on the country and the route. I don't live there but have family. They say some train lines are comparable to a plane ticket. Some are cheaper, some are more expensive. The most expensive ones are usually the ones that cross national borders. Though overall the train will always be considered a tiny bit more expensive, it will also be a lot more comfortable. Since the airlines with tickets cheaper than trains usually offer the tiniest seats with zero amenities.

Some countries are better than others as well. In England, trains can get you almost anywhere relatively comfortable and fast (yes, some caveats exist). Spain is rather good in the central regions but many places have spotty coverage. While in Switzerland you can go virtually everywhere except the alps without ever sitting in a car.

The big comparison, that most people fail at, is not just the price of gas. Cars are taxed heavily almost everywhere in the EU. And the tight emissions regulations also require a lot of regular maintenance. So the train will always be more expensive than just the gas, but not more expensive than the bigger picture of car ownership. Plus, if you drive, you're the sod stuck driving the whole way.