The increase of tank car trains carrying oil in North America has increased safety concerns especially with regards to transit through populated areas in the aftermath of the 2013 Lac-Mégantic accident. Among other things, this has further led to the desire to have accurate models to predict the dynamic behavior of tank cars for assessing derailment safety. In pursuit of this, the FRA has funded research to develop a tank car model which includes the behavior of the fluid inside tank cars, so that the resulting forces can be accurately depicted in multi-body dynamic modeling codes without the use of computational fluid dynamics. Prior analytical research on linear fluid sloshing has been adapted to create a mechanical model (pendulum–mass system) that estimates the lateral fluid motion and resulting forces which can be added to a dynamic model of the tank car. The developed model parameters are compared to other works in which mass and frequency parameters are derived. The model derivation, equations, and comparison to test data are included in this paper. Parametric results are provided showing trends associated with different levels of fluid fill.