Due to environmental changes, extreme weather, and other factors, maintaining effective insulation of the rail over time is challenging, leading to a complex distribution of rail potential (RP). Excessive RP significantly impacts the operation of urban rail transit (URT), so this paper aims to research RP distribution under different insulation sections. Firstly, distributed parameters are inverted into lumped parameters to establish a dual π-circuit equivalent model based on the structure of URT. To simplify the calculation, a unified chain model of the reflux system under regional insulation degradation is developed. Two new nodes are added to the model to simulate insulation degradation. Subsequently, an RP calculation method is proposed based on the circuit theorem, and the conductance sub-matrix for the case of insulation degradation is derived. Finally, RP simulation is conducted for both single and multiple long sections, using Guangzhou Metro Line 2 as a case study.