Here is an interesting video on how tractive effort is calculated and some of the myths surrounding it.
https://www.youtube.com/watch?v=_Q_XCF_8fTg
Mark Huber (Hyce) is a steam locomotive engineer and works at the Colorado Railroad Museum in the mechanical department. He is very knowledgeable in steam railroading. He has published many videos on the subject. He also has written for Trains and The Drive magazines. After this video you may want to follow the additional links to additional videos from him.