Money is nothing; RailFanning is Everything

While it's true that TALGO coaches are shorter in length, it's also true that they have only 4 wheels (in place of the usual 8). That's where the concept of the unconventional "Jacob's bogies" fits in. Now, if the length of coach is increased, the weight will also increase proportionally. Then 4 wheels may not be sufficient to hold the coach or else, the weight on wheels may go out of limits which the P-way or the suspension can handle (not using the term "axle load" here).
Making the coach shorter reduced the length of the rigid wheelbase, thereby allowing the coach to negotiate curves at higher speeds. Also, TALGO coaches do not have axles. Wheels are independently installed...
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and suspended. The suspension goes right up to the roof and is supported by a column. As a result, the wheels can rotate at different speeds.
This concept is unlikely to work on a 40-45 tons, 24 metres long LHB coaches. Given the weight and length, the coach would require 8 wheels. Where will those columns be installed for supporting 4 extra wheels?
TALGO coaches can work on different gauges too (broad gauge, standard gauge etc), which makes them highly adaptable. The presence of all these features is what made IR to go for TALGO in the first place! :)

It's 80 kmph. :)
I should have been a bit more clear about what I meant though. Beyond 80 kmph, the effect of the coach's cross sectional area becomes a significant portion of the total resistance to be overcome. It does not make a big difference to the ride quality or hamper the train in achieving a higher speed, but the locomotive has to put in more energy to overcome the resistance. But yes, it is limit only as far as energy efficiency is concerned, not ride quality or feasibility of high speed runs.
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If the air resistance from head-on-wind is considered, then depending on the specific conditions, sometimes, normal non-streamlined locos ( like my favorite WAM4 :P ) or coaches with high ground clearance, can also move at speeds of 200kmph +. This is because, the fast moving locomotive compresses the air in front of it ( similar to a falling meteorite, where this compression is called ram pressure). This compressed cone of air usually acts as a streamlined face, reducing the air resistance by a good amount. But for the case where the winds are hitting the train at an angle, directly hitting the side walls of the coaches, the coach ground clearance, height etc becomes an issue and this truly restricts the normal operational speeds and this will affect ride quality as well.