It certainly used to be that way here in the UK too, and probably for the most part still is. I think the bottom line is that large diesel engines are like a magic schrodinger machine - if they're running, you know they work, but the moment you stop them, there is uncertainty as to whether they'll start back up again without visiting the workshop first.
I used to trainspot at a nice vantage point that overlooked York yard back in the late 70s whenever I got chance, and every engine you saw sat waiting was almost always idling away. In fact, if I recall correctly, I only ever saw 2 engines start up in that yard, a class 20 and a very claggy deltic that was hit-or-miss on starting its second engine. Everything else you'd have had to have been there at 4am to see it start up