The only real difference between the old rule book(s), like the Consolidated Code and the SP's rule book, and the GCOR is that the usage, style, and syntax of the language was cleaned up considerably in the GCOR (never mind the omission of train order and schedule rules). The old books were full of run-on sentences, confusing series, and convoluted trains of thought. The GCOR is largely devoid of problems like these.

And yes, both were written in a certain amount of blood. The other big consideration was the loss of money in the captial investment of the equipment and physical plants damaged and/or destroyed.