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This paper presents the development of detailed simulation models to study short-circuit protection of on-board dc auxiliary systems in a rapid transit train. As the investigation of protection solutions during the testing phases is costly, it is necessary to perform detailed protection analysis earlier in order to detect potential protection issues. This requires modeling the behavior of batteries, auxiliary converters, and the detection and arcing mechanisms of protection devices in detail, as shown in this paper. The protection device under consideration in this study is the molded-case circuit breakers (MCCBs) equipped with adjustable thermomagnetic trip unit which are widely used in the railway industry. To maintain battery system availability, they are selected, sized, and adjusted to avoid false tripping during emergency conditions. However, they must also be sensitive enough to detect the limited available fault current which is specific to rapid transit train dc auxiliary systems. Tolerances on the detection time of thermomagnetic units may be unacceptable as shown in this study. In order to overcome this problem, electronic trip unit assistance to conventional thermomagnetic-type MCCBs is proposed and the requirements for such a device are evaluated using the detailed simulation platform. [ABSTRACT FROM AUTHOR]