High capacity (10 GW, for example) passively cooled non-superconducting underground high voltage direct current electric power transmission lines (100) of very low loss (1% per 1,000 km, for example) and competitive cost. The transmission lines (100) include segment modules (101) linked together with compliant splice modules (102) between the segments (101), typically installed in a protective conduit (103). The segment modules (101) include relatively rigid pipe-shaped conductors (117) insulated by pipe-like solid insulating layers (131) to form segment modules (101) that resemble pipe. The segment modules (101) are linked together through radially and axially compliant splice modules (102) to form the transmission line (100). There are preferably wheels (300) deployed to ease insertion and removal of the assembled segment modules (101) and splice modules (102) into the conduit (103), to center each segment module (101) within the conduit (103), and/or to provide motive force and/or braking to allow the assembled segment modules (101) and splice modules (102) to be installed on a slope.