Alas, I still haven't found any that let me do the same for trains and buses. GoEuro already lets you search, but you have to choose a specific destination.
Finding cheap train fairs is indeed a puzzle sometimes! It helps a lot to have some familiarity with the major European train companies, the major high-speed connections, and how to book the discounted tickets in advance (incidentally, Amsterdam → Berlin starts at €39,90 when booked two months in advance). Of course seat61.com is useful too as a general source of information.
What I really miss is a tool that shows me destinations that can be comfortably reached by a combination of day-time and sleeper trains on dates that still have discount tickets available. Nothing beats waking up early in the morning in Vienna or Munich in a sleeper train; it's like getting a free day of travel, and you arrive fairly well-rested instead of weary from travel. I'm not doing anything useful in my sleep (besides sleeping), so I might as well spend that time travelling.
However SkyScanner just updated their capabilities to include Train travel on their booking similar to GoEuro. I still believe SkyScanner to be the best travel website for finding cheap flights and also includes the option to just search a country name like "Spain" or even "Everywhere".
I'm excited to see SkyScanner merge the technology of including buses/trains with the ability to explore non-specific destinations.
https://techcrunch.com/2018/04/03/skyscanner-adds-train-trav...