It's not shown but I assume that class RollingStock defines method getChassisCount() and fields chassisCount and maxWeight.
Your mock is not working because you have mocked RollingStock at the start of your test: @Mocked RollingStock rollingStockMock;
Because of this when your test calls car.getChassisCount() you are calling a mocked method and you are getting the default return value for a method returning an int, which is 0. Removing this mocking enables your test to pass.
Note you have the Assert.equals parameters mixed up. The expected value should be the first parameter and the second should be the actual parameter. This will make failed test cases easier to understand.