Sticking my neck out here I know, but heavier trains would in no way require track replacement. The margins track stress is calculated to are large enough to accommodate a greater excess mass than that caused by a slightly heavier train.
Not to mention, support location and structure is determined by the prevailing forces acting at that particular point in the layout, not the mass of the train moving over it. I know, slightly greater mass x same force = bigger effect, but it's still well within the limits of the existing track.