This work presents research in the development of medium frequency communications systems that the National Institute for Occupational Safety and Health (NIOSH) is performing in support of the 2006 MINER Act. In this work, a 280-m twin-lead (30-cm separation, 725-ohm) transmission line (TL) was constructed and used to measure complex input impedance and line currents resulting from open and shorted terminations at medium frequencies (MF). The line was placed in free space and positioned close to a lossy earth surface. This 3-conductor system (two wires and earth) was chosen to be a starting point to investigate the multimodal nature TLs may possess inside a coal mine. Although the TL was constructed to be physically isolated from the ground, measurements indicate the existence of common mode (CM) currents which are characteristic of a three-conductor system; hence, the ground affected the MF propagation characteristics. In this paper, full-wave computational models using moment methods are compared with the experimental measurements as a way to determine the origin of the CM currents. In the future, related experiments will be performed in underground mines in an effort to predict how multimodal MF signals will propagate in the presence of similar multiple-conductor systems.