Trains must be able to carry a load of 600 or more people and their belongings, repeatedly accelerate them from 0 – 60 mph in under a minute, and travel hundreds of miles without needing to spend more than a few minutes to refuel. Within existing laws of physics, it’s not possible to power a train that can meet those requirements by coating it with solar panels; nor are any hydrogen fuel-cell powered vehicles currently able to meet those requirements. If you know of any, please let us know. The technology still has a ways to go to advance beyond prototype stage; read about the results of VTA’s hydrogen fuel cell bus demonstration program (pdf, 1.3 MB, June 2006). Alternative power sources can power Caltrain; just not directly.  Solar panels or fuel cells can feed power into the general electrical grid that powers the electrified trains through an overhead catenary.