Is it theory of relativity?

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Dr. Albert Einstein says that, " 2- trains for a passanger in train-1 moving in same direction with constant velocity to second train seem to be still" , since both r mutual related to each other. Provided that the person is attached to the train. But If we consider the fly or bee that flies inside the train without touching any ground source .. seems to be fling with the train in same compartments. How it could be possible? Is that bee flies with respect trains velocity? Not sure Is that bee able to fly with train 100 km/hr or it has capacity to fly? not possible How can this happen? Is this theory relativity?