Great stuff! Thanks to all for replying. I am a little overwhelmed but much food for thought here.
Clearly there had been ups and downs in locomotive building, and Brush's later experiences were one of those, rather than a fall from some golden age.
Market changes also matter a lot. I assume the move to MUs is partly cause and partly effect: no locos means you are locked in to one sector, so you focus on that.
GEC and Weinstock. As someone said of them regarding their aerospace work they "never willingly over-resourced any project". Seems the train business was the same.