system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the smokestack of old steam-powered locomotives have that odd narrow-wide-narrow shape? Why isn’t it just straight like a pipe?
gpt
Spark arrestors.  So, when you burn things, you can end up with flaming little embers escaping with the steam.  Those that exit out the chimney could land on brush near the train and start fires.  So chimneys had these little mesh grates called spark arrestors to catch the embers and hold them in place preventing them from escaping and causing fires.  But that causes a build up of residual material in the chimney, reducing pipe area, which reduced the exhaust and subsequent power of the engine.

So those wide brimmed areas were designed to funnel the smoke, and the embers with them, to the side where the mesh grating would catch the embers and deposit in in the wider brim.  It'd have to be emptied out occasionally but prevented buildup in the main chimney chamber.   


It's basically a bucket to hold stuff to prevent fires while also not clogging the main exhaust.