Best solution for making a LAN with 2 untwisted lines
Having done radiated noise tests by railroad passing coaxial cable TV residential distribution lines, in the 90's, I would avoid baseband and VHF and look for a slotted aloha UHF solution 0.3~3GHz, TDM SCADA solution with E1 data rates of 2.048 Mbps and up with high res video compressed with MPEG but only 60 frames per second  shared by 20 transmitters. Controller requests interleaved frames from each source to asigned time slots or more for one channel requiring more attention. Low speed controller can be sent securely with encrypted spread spectrum for time slots of each unique address that responds. I don't have current commercial solutions, but I would avoid baseband and VHF due to traction motor noise.  Many tradeoffs between cost, reliability, security and susceptibility to interference.