The development and implementation of a Prolog expert system using computer graphics is reported for the on-line monitoring and control of the 50 Hz single-phase overhead current collection catenary system currently in operation in Hong Kong. Running as an on-line consultant on a personal computer, this expert system allows operating personnel to perform intelligent network switching on a graphic picture representing the whole of the catenary line system. Users are able to find out effects of their own choice of switching sequence by examining graphic displays. Valid switching brings about changes in display configurations and colours to record changes in the on/off status of circuit-breakers or isolators, or the relocation of supply points. The expert system also checks against any invalid switching which causes a violation of interlocking rules, an isolation of overhead-line sections from the supply, or a phase-to-phase short-circuit of the utility system. Work to develop a methodology of evaluating voltage across voltages across crossovers which run between parallel overhead lines is also reported. A crossover with excessive voltages across it should not be used to divert a train owing to the risk of damaging insulators across the crossover. An accurate evaluation of crossover voltages usually involves the solution of a large set of simultaneous differential equations governing train timetables, controls, movements and power consumptions. It is shown how train consumptions may be referred to their nearest static busbars and the resultant admittance matrix does not include any moving busbars (trains). This ‘reduced’ matrix occupies only a very manageable memory for either a full power flow solution or any other approximate method of voltage evaluation