Another (very quick) take on the caustic soda locomotive concept based on this comment on my last postcard about what a version with swappable boilers might look like.

The idea is that instead of pumping out the caustic soda to dry it, they would instead unbolt and lift off the boiler, probably using an overhead or gantry crane, and replace it with an already-dry one. The dilute one would be inspected, and placed on a concrete containment pad where it could be connected to a solar steam generator, so the superheated steam could dry the caustic soda. This is actually pretty similar to how they apparently did it historically, except using a coal boiler and obviously without removing the boiler from the locomotive.

Ideally, this would be a bit safer as the boiling hot caustic soda would remain contained for the majority of the time, with less risk of spills during the drying process, and the extra boilers and frequent inspections could help prevent corroded parts from disabling a locomotive and stopping a train line. It might even be faster, depending on how complex the hookup process is.

In the end, it’s probably not a whole lot more practical, but I really liked the idea (suggested first by Carrier_Indomitable over on r/trains, and then with some cool visual details by @WaterWaiver@aussie.zone on the last post.