a lot of control theory goes into steam train locomotive, you don't say it explicitly but the way people typically make the same reference they evoke the image of a simple steady state steam engine, and insinuate that people compared brains to whatever was the novelty. I don't think that was ever the case, and comparing the brain with the intricate feedback systems, regulators and more general control theory facets of an actual steam train locomotive is a lot more apt than people make it out to be as if people were comparing it to a simple steam engine proper (i.e. not a locomotive).

also if you look at lifeforms with and without brains, and lifeforms that do or don't do locomotion, there is a clear correlation between mammals, birds, reptiles, spiders, insects, ... which have brains and are motile, versus plants, fungi, ... which don't have brains and aren't significantly motile.

the moment you need to move (not just grow in this or that direction) you need a lot of things: muscle control, inverse kinematics, interpretation of the environment, speedy reactions, routing, planning, memory, ...