I heard that they use diesels in areas with long tunnels (FRC as an example) so the accumulated stack exhaust from the steam locomotive under a load captured in the confines (yes they have vent flues in some of their tunnels) of a tunnel won't kill the locomotive crew. The steam locomotive engineer eases back on the throttle, the steam locomotive coasts and the diesel pushes it through the tunnel with lower exhaust stack output. When they are through the tunnel, the steam locomotive engineer opens the throttle and takes up the load again and the diesel is eased back. Yes I heard that the steam locomotive crews have breathing apparatus but the diesel is a big help in that situation. Was I misinformed? Keeping in mind this is a UP practice with their own steam equipment. What the rules are with a "foreign" locomotive on their right of way on the rare occasions that that would occur would be something else. What other railroads do as a matter of practice, I don't know. Someone with experience on other roads could contribute valuable information here.
:)