Renewable geothermal energy is an energy source that can be used for heating and cooling purposes. One common method of harvesting this energy is using engineering structures in contact with the ground, such as structural piles, diaphragm walls or tunnels (with geothermal loops integrated into the tunnel lining/invert (tunnel ground heat exchangers (GHEs))). Moreover, the heat generated in the transportation tunnels (primarily from engine brakes) can be exploited by nearby borehole heat exchangers (BHEs), and can enhance their thermal performance in heating dominant climate conditions. This study investigates the thermal interaction between a tunnel GHE and a BHE in the tunnel vicinity using a newly developed 3D detailed numerical model, and highlights the influence of this thermal interaction on enhancing the thermal performance of BHEs considering the effect of groundwater flow. The dual heat transfer process between the tunnel GHEs and the air in the tunnel, as well as the surrounding ground, is also investigated in this paper. Numerical results show that thermal interaction between the BHEs and the tunnels enhances the thermal performance of the BHEs in comparison to a BHE with no tunnel nearby. This performance improvement is also boosted up by groundwater movement for downstream BHEs. The major driving factor in this thermal performance improvement is the heat exchanged between the tunnel air and the surrounding ground.