If 3rd rail is so unsatisfactory, why do most Metros, including new ones, use it?
Such comparison discussions never seem to feature any sensible cost anaylsis of the two approaches. At installation, the 3rd rail goes down notably quickly, without the extended time, cost, and heavy civils work that overhead does. I actually watched in the mid-1980s the laying of 3rd rail on the Stratford to North Woolwich line. It virtually seemed to go in over one weekend - works train propelled by an 08 shunter, rails offloaded directly to location, bolted down, team ahead screwing in insulators, move forward one rail length to the next one. No civils, no bridge lifting, done. Cost maybe 5% per mile of what it took on the Goblin with overhead.
All the stuff about more lineside substations, but on a conventional 12-car 25kV emu there are 3 substations, transformer and everything, under each train, one per motor coach. This one is sometimes rebutted by stating that things have got more efficient with power electronics. Well, so have lineside structures benefited equally.
We may contrast two similar "intermediate" systems in Britain, developed at broadly similar times, the Tyne & Wear and the DLR. One went for overhead, the other for 3rd rail. Which in retrospect was the better solution?