Transmission expansion planning (TEP) problem is a study of identifying new transmission lines that need to be added to an existing electrical system, and researchers’ interest in TEP problem is increasing. There are many different test systems in the literature that researchers have chosen to test and compare their solution methods. In this study, the TEP problem is classified according to coordination with other planning systems, test problem types, and solution techniques. While classification according to solution methods and coordination with other planning systems is an updated version of past literature studies, classification according to test systems has been comprehensively discussed for the first time in this literature review. In addition to the first-time classification of test systems, a detailed review and classification have been made for the IEEE 24 test system, which is found to be the most used test system in the TEP problem literature. The IEEE-24 test system is originally classified considering new transmission line alternatives, new stations, various demand incentive policies, generation alternatives, generation quantities, load amounts, and transmission line capacities. With this study, researchers will compare their proposed techniques with past studies in the literature, and their proposed methods' differences and specificities will be identified.