This is why "Precision" is Newspeak in so called Precision Scheduled Railroading (PSR). All but a few engineers and conductors for whom trains can be reliability scheduled are on-call as a train is built up to the arbitrary number of 150 cars or whatever. Whenever it's of the "precise" size it goes, unless one of those two are unavailable.
And since no one can predict when a substitute might be needed, it would take many more spare engineers and conductors to cover for those out on a sick day.
This has to be why the railroads are fighting so hard against this apparently trivial benefit, despite the bad optics etc.
