I seriously doubt he (the diesel coach) was idling for "90 minutes." Exaggerated I am thinking.
.Diesel engines must have warm Up time because they're ALL lead-sleds without adequate warm up time. Diesel engine Class As, are likely to have air brakes as well. ALL air brake systems need to be adequately charged with compression before they'll even operate.
.Unless you've owned and operated a diesel you're probably unaware of these facts.
All my coaches, and most of my cars over 30+ years have been diesels. None perform at all without a good 5 to 10 minute engine warm up period of start up time elapsed. Especially in the Wintertime.
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