Well I'm going to talk about UK steam locomotives. In the past, they used to try attaching all sorts of gadgets and devices in an attempt to improve the depressingly low thermodynamic efficiency of typically 6% and save a bit of money by reducing coal consumption. Usually any improvement was minimal and was offset by the temperamental behaviour of the devices, reduced locomotive availability and increased maintenance costs, so they'd strip them out and go back to 'stock' locomotive.
(But users speak favourably of the Greenify app....there's a recent thread somewhere in the forum.)
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