The factors contributes much in attaining good and efficient burning depends on the design of the coal fired steam boilers. Obviously old locomotive boilers were designed for lumps of coal to be shoveled in or fed in with a power stoker. In more modern boilers pulverized coal is used and they probably run a higher temperature than older types.
Quality of coal - low ash content, low moisture content are important, but the more important is calorific value, which depends to some extent on ash, moisture and "fixed carbon" content.
Frequently steam raising coal is not of the highest quality if it comes from a coal province with coking coal. Steaming coal often comes from the tops of a coking seam and has higher moisture and ash content than the seam average. The best coal goes to the coke ovens, the rest goes to the coal fired steam boilers. However some mines produce exclusively steaming coal and some of these have low ash and moisture contents. Anthracite are the best steaming coals in general, they have low moisture and volatile content and the best anthracite have low ash as well.
Another coal quality is ash fusion temperature. If the ash produced by the coal melts at a low temperature, ash can deposit on boiler tubes and reduce heat transfer. A high ash fusion temperature means this does not occur to a great extent and the ash flies out to the electrostatic precipitators and baghouses where it is mostly removed from the exhaust stream.
Another factor is the correct amount of air fed to the coal fired steam boiler. Too much and the heat is used in heating air, which could better be used in boiling water, too little and combustion is incomplete, leading to smoke and money spent on fuel going up the stack.
All of the above are the factors that contributes much in attaining good and efficient burning in a coal fired steam boiler.