A modular rail assembly for securing a panel in a door or window, comprising a rail framing member having vertically-oriented sides interconnected by a partition plate to form first and second receiving channels on opposite sides of the partition plate, the second receiving channel defined at a top side by the partition plate and on opposing sides by receiving channel walls, and a rail base extension slideably secured within the rail framing member second receiving channel. The rail base extension includes vertically-oriented sides interconnected by a partition plate, the rail base extension sides having mating surfaces on opposing external surfaces thereof for mating with internal surfaces of the rail framing member second receiving channel walls. A fastener secures the rail base extension to the rail framing member, and a panel is secured along a longitudinal axis thereof in the first receiving channel of the rail framing member.