Solar panels on the train car roofs would utterly fail to propel the train, even at noon. So, the energy has to be concentrated somehow. Batteries are heavy, an

Solar panels on the train car roofs would utterly fail to propel the train, even at noon. So, the energy has to be concentrated somehow. Batteries are heavy, and also involve a pair of chemical transformations. Hydrogen is the single most energy-dense (per unit mass) medium. But hydrogen is not best compressed, but liquified. There might be some value in compressing it on the way to liquifaction. Liquifying hydrogen consumes some energy too.