GF you are partly correct, however the testing we did showed like you say a hig level of CO as well as other things not so good for humans and other living things. Also there is a time limit in which track haulage diesel equipment ( man trips and locomotives) is permitted to run, IE you cant sit with it idling and the heater on on the main line track or off a side track waiting for clearance to move.
Equipment ( scoops, end loaders, roof bolters ect.) used to move material is run as needed to be, how ever it must be used in only intake air and then only so the fumes can be directly sent to return air going out side. Underground diesels are an animal all unto them selves, and dispite what you see about guys driving a dodge, chevy or ford diesel trucks underground the motors must meet very stringent guide lines and are not what you find out on the highways.
Though Metal Non Metal laws at one time were much more lax than coal laws for underground diesel usage, the new diesel regs that have been brought in to effect, are both pretty much in line with each other. Fuel storage underground is an whole different matter
