File:Caustic Soda Locomotive at a Solar Drying Sation.png

English: Soda locomotives were a type of fireless steam locomotive that barely made it out of the prototype phase, where the boiler is surrounded by a tank of ‘caustic soda’ (usually one of several possible chemicals), which generates heat when mixed with water. The heat produces steam in the boiler, which is used to drive the pistons, but instead of being released, its condensed and added to the soda to create more heat. This goes on until the soda gets too dilute to produce more heat, but it can be ‘recharged’ by drying it out again.

These never really took off because it took more coal to dry the soda at the station than to just run a conventional steam locomotive, and electric trains quickly came into their own and filled the niche of quiet, low-pollution trains for inside cities and tunnels.

I feel these could pair well with solar steam generators (another late-1800’s design) stationed along the tracks, to create analogue, solar-powered trains. These could run on existing unpowered tracks, without requiring any new electrical infrastructure, just the isolated drying stations.

The train crew would just exchange wet soda for dry and start again (it looks like that took about 45 minutes). The interesting thing is that this arrangement could be asyncronous – the station can dry out the caustic soda, then store it for when the train shows up. The train can run on cloudy days or at night, as long as they get enough sunny days to dry out big batches of soda at the stops along the way. And the solar concentrators can be huge and optimized for their location because they don’t have to move.