My favorite train ride in Bulgaria covers the stretch of track between София (Sofia) and Мездра (Mezdra) along река Искър (the Iskar River) and through Искърското дефиле (the Iskar Gorge). Wanting to take in the scenery at a slower pace, I recently spent a couple days walking the stretch between Лакатник (Lakatnik) and Люти брод (Lyuti Brod). I'm too much of a hack to do the place justice with photos, but you can get an idea of the beauty of the place and how enjoyable my two walks were.
