If you really want to do an accurate job of determining the number of cars of coal needed to generate power, you can look at how that is done on my Chemical Engineering 357 class web site at The University of Texas at Austin under part III lecture notes.

http://www.utexas.edu/research/ceer/che357/PDF/Lectures/intro_air_pollution.pdf
 
These the notes I use from my lectures, so there are many comments provided in class.  This course is designed for senior chemical engineers, so there are many assumptions made about the knowledge base for the students.  In calculating the amount of cinders, you can calculate the amount of particulate matter that is emitted througth the stack, calculate the amount of ash in the coal, and the difference will be the amount of bottom ash (cinders).  The example illustrates the process with a typical coal.  The BTU content and ash content for coal vary significantly for coal mined in various parts of the country.  To obtain more examples of various coals and lignites you can look at the coals I use for project 3 that my students have to complete.  These are for coals mostly in use today.
 
http://www.utexas.edu/research/ceer/che357/PDF/Projects/Project3/ChE%20357%202009-project%203%20fuels%20and%20stacks.pdf
 
Most likely a lot more than any of you want, but just to let you know there is a way to determine the amount of coal needed to produce a given amount of power.  Also needed is the efficienty of the power plant.  Coal plants typically have a maximum efficiency of 41%, with older plants being much less than that.
 
Cyril Durrenberger