While the concept of a gravity train isn't beyond comprehension, it's certainly fun to think about. A short video about gravity trains concluding with an engineering challenge can be found here. If you rise to the challenge, or simply have any interesting thoughts or ideas, feel free to share them here.
Three episodes of The Engines of Our Ingenuity, a program airing on National Public Radio's Houston affiliate, touch on topics related to gravity trains: Gravity Train, Gravitational Transport, and Tube Trains. More detailed information about various aspects of gravity trains can be found in the end notes and references of these episodes.