According to what the C&S was putting out, they were taxes levied on each and every boiler within the State of Colorado. It sounded like some sort of annual levy.
I don't know, When the C&S said that, I was fresh out of High School and knew better than to question the owner of an object who had more money than me and with plenty of cutting torches and employees to wield them.
I do know that Colorado has its own boiler laws and State boiler inspectors. Locomotive boilers passed by Colorado inspectors can run intra-state but not necessarily Interstate. Of course the C&S was an Interstate Carrier, not just Colorado.
Rick