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Thanks!
There is no particular problem with hauling larger coaches ( Most of our locos are more than capable of handling such loads) The reason for making shorter coaches is the need for making high speed coaches. You can search here on IRI for some posts explaining the reason why longer coaches of the Indian style pose problems for higher speeds. And also, in the Talgo rakes, each coach shares a bogie with the adjacent coach and there are no couplings of the kind we see in our Indian rakes. All this makes short coaches necessary and also helps them in traveling at higher speeds on curves.
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You are right about the coaches being lighter, but usually in reserved coaches where there is no overcrowding, the total weight of the passengers is only a small fraction of the coach's weight. In an average Three tier AC coach, the weight of the passengers is barely 10% of the total coach's weight, and the corresponding increase in the power required from the loco isn't much. In this scenario, the Talgo will still be efficient because of the lighter coaches. While a 3 tier AC coach with 72 passengers weighs about 50 tonnes, the same capacity is possible with 2 Talgo coaches, which together weigh about 26 tons. So overall, it is still better for the loco.
That said, not everything is rosy about the Talgo coaches. Platforms have to be modified, maintenance facilities have to be setup, rake augmentation or modification is a big hassle among several issues. But, it's a good start at least.