Coupling phenomena between two parallel multiconductor transmission lines is usually dealt with using modal analysis techniques. However, when the interaction length is small compared to the wavelength, a simpler quasi-stationary approach can and should be employed. Using the latter approach, the general problem of the determination of capacitive and inductive differential voltages between conductors of the victim transmission-line circuit is addressed, resorting to the formalism of the per-unit-length series impedance and shunt admittance matrices. An application example concerning the interaction between an overhead power line and a two-conductor railway is presented. Results obtained are interpreted and validated using electromagnetic field calculations. The dependence of the differential voltages on the frequency, on the interaction length, and on the lateral distance between interacting transmission lines, is analyzed and discussed.