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Bullshit. For someone with so little depth of understanding, you certainly have the air of an expert.
Firstly, The entire national road network is being upgraded to 70-T-R class, and I believe the process is almost completed in the western regions. So no, you wont be spending lakhs of crores of your money on upgrading the bridges.
Secondly, The bridges and tracks, infact everything for Trains are designed to carry much more than mere 50 tons or 70 tons. I can pull out official data, if you want me to back it up.
Thirdly, Arjun has less ground pressure than a T-90, despite the difference in size and weight. Which means, the Arjun can go where your dear T-90 can't.
Fourthly, The T-55, up-gunned with a 120 mm cannon can do the work of a T-90, as per your logic. Why go for the T-90 then?
If the army wants to go for the western concept of armored operations, and emphasize crew survival, it has no choice but to go for heavier tanks, or a smaller crew component.