In any type of locomotive fuel is turned into drawbar energy that moves the train. It doesn't make any difference what type of fuel is used, coal, wood, petroleum, old cooking oil, methanol, or whatever. The efficiency that energy contained in fuel is converted into work can be analyzed by using thermodynamics.

In the January-February and March-April, 1993, issues of "Locomotive & Railway Preservation" magazine William Pettijean performed an in depth analysis of the theromdynamics of steam locomotives vs. diesel locomotives. He showed that a diesel locomotive is many times more efficient at converting fuel energy into work than is a steam locomotive. This difference is inherent in the designs of the two types of power. He concluded that the replacement of steam by diesel power was inevitable because of the much higher efficiency of diesel locomotives.

This two-part article was expanded into a book several years later. If you want to know more,
read the articles or find a copy of Mr. Pettijean's book (sorry, I don't have publisher information available).

fk