بسم الله الرحمن الرحيم
No, if you have even least chance to offer salah standing then it is mandatory to offer salah by standing in train and flight as qiyam will not drop.
Note: The answer is correct, but if you offer salah in the bus or flight sitting due to some reason then you will have to offer the salah again.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.