Nuke and geothermal are base-load production, much like the coal-fired steam plants elsewhere in the country. They are that way for exactly the reason you describe - they can't change output significantly in a short period of time. So they can't be easily or cheaply put on standby when solar in CA makes a large dent in the demand curve (when the sun's out, especially in the desert, where the big solar plants are). Wind likewise, though more variable (CA doesn't have the nearly constant wind over large areas found in West Texas or the Dakotas; it's more localized and varies greatly over the course of the day). Look at the CalISO's charts for examples: [www.caiso.com]

The funny railroad for power storage has been discussed here before. In principle, it should work, but won't store even nearly as much power as pumped hydro (or deliver it for as long a period) unless you have a whole bunch of them. OTOH, as noted elsewhere, there ain't much water in the desert, so this might just be a useful scheme. Emphasize MIGHT. And for safety reasons (Caution: Automatic Train May Start At Any Time!) it almost certainly will be well-isolated from public access - but what about the tortoises?