Trainline
Trainline is an app that lets you book tickets on trains (and buses!) across Europe. Besides being popular in its native UK, Trainline’s coverage includes Germany, France, Italy, and Spain. Besides reserving tickets, you can also stay up to date on your trip with useful features such as departure times, platform information, and tickets stored on the app. One useful feature is that the app lets you search for routes that will specifically pass through (or avoid) certain cities so that you can nail down your itinerary even better.
Trainline is available on both the App Store and Google Play.