Over the weekend i went to grantham & stayed at our friends house, But On the way there we arrived at the train station 55 mins too early the wife's fault?
Anyway i bought a coffee for us both & we sat down on the seat next to a train that had been left standing, After 25 mins my wife mentioned to me that the train that was about 8/10 foot away from us had been left on since before we got there!
When our train arrived the stationary train (Diesel fueled) was STILL left running with the lights out inside & no access to enter so it had in fact just been left on! for at least 50 mins & they was three carrages each with a exhaust pipes around 8 inches wide & belting smoke out of them.? Thats a lot of wastage dont you think.? & why would they just be left.?
Your truck's engine is designed to pull a load and has power developed over a reasonable rev band unlike the local train where the engine drives a generator and the rev band is very limited. Power to drive the train comes from electric motors mounted on the axles. The generator also powers the train heating which is needed even when sitting idly at the station.
I built the control gear of the diesel locos that run on London underground for maintenance purposes at night. They have to run for at least an hour idle or 20 min under load above ground before the emissions come within limits to let them go underground, once within spec they can be used all night if left running. Let them cool down and you have to start the cycle again.
to idle for long periods - it's a safety measure. Idling the engine maintains pressure in the braking system, and operating rules require drivers to leave the engines idling when the train is stopped on a traffic line.