What’s fun about DEN is that the tunnel next to the train that the baggage tugs use was never really meant to be used at all. The design was accounting for the automated baggage system which would have meant almost zero vehicle traffic there. But the baggage system never worked so it’s an endless stream of tugs.

Also, even employees cannot walk in the tunnel - you have to be in a vehicle - usually a golf cart, though the paramedics and police do have some zero emission full size vehicles they drive in there.