I have seen two other metros besides the one in Delhi. The London metro or tube, as it's known, is very extensive but old and very expensive. The one in Tehran, Iran is quite new but more crowded than the one in Delhi. Delhi metro is the best.
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Yup Delhi metro is much much better than many others across the globe. However its network is too small. Once it grows in size I guess it would also have problems. However it is amongst the very few metroes in the world which is making a profit.