Abstract: A modular base side bearing assembly has a cage with upstanding side walls defining a bearing cavity and a central bottom opening to receive a modular base on which a bearing element supported on a central base portion thereof. The modular base has externally visible tabs identifying a force level corresponding to the modular base, in which the thickness of the central base portion corresponds such force level. The modular bases are interchangeable with modular bases having central base portions of differing thickness, or zero-thickness, such that the same cage, bearing element, and wear cap can be used with different modular bases to provide different force levels. The modular base is movable vertically relative to the cage to accommodate uneven mounting surfaces.