Does the diesel price ($3.97) include road taxes, which wouldn't apply to railroads? Thought about that after I'd posted.
The $3.97 is the average for highway grade price, so it would include the tax.
If I read Chapter 1 Fuel Taxes Page 7 of the following correctly, that tax applies to trains as well.
Publication 510, Excise Taxes
http://www.irs.gov/pub/irs-pdf/p510.pdf