My favorite method of overland travel in Europe is bus. Flixbus is my favorite bus company because their service offers free WiFi and a power socket for each seat. The Flixbus fleet of buses are modern and spacious so you have plenty of space to either relax or play on your smartphone.
When traveling by train in Europe you must plan days and weeks ahead to get reasonably priced train tickets. Often planning days and weeks ahead is not possible as you need to be flexible to enjoy your travels to the maximum. With Flixbus you can find cheap bus tickets the day before and even on the same day of travel.
For train, bus and air travel in Europe GoEuro is a good method of checking timetables and the methods of travel available. I usually check Flixbus first and if they do not serve my route I will check the route on GoEuro. For example between Estonia and Latvia there is no Flixbus service or train, GoEuro will show a different bus company which serves this route. GoEuro is useful for finding different methods of travel that are possible on the same route. It also shows the different timetables and prices which is valuable information whilst traveling. If you find an option which suits your requirements you can click through to the relevant site and book online.