My guess is that there are economies of scale in running fewer trains that are longer. Replacing 3 trains x 3 locomotives each with 2 trains x 4 locomotives each reduces the fleet requirement by 1 locomotive.
Channeling Hunter Harrison's precision railroading business model. Moving freight on a schedule. Sad thing is I think we will see more rolling stock mothballed and trackage railbanked even with a quick reopening of the economy.