I see flies, in a train moving at enormous speed and confused how could they fly parallel with the train at the same speed. I admit law of motion applies here that a thing over a moving object acquire the same speed, but here the is not resting, it's flying along, so gradually it should lose its speed.

Are you talking about a fly that is inside the train, and the fact that it buzzes around as if the train was standing still? As far as the fly is concerned, there is no difference whether the train is going 100 MPH or standing still. All velocities are relative - that is, there is no such thing as an absolute velocity, and consequently all laws of motion are with respect to an assumed rest frame of reference - in your case the train is the frame of reference for the fly, so it matters not how fast the train is moving relative to the ground. It's the same principle that allows you to jump up in the air while on the moving train and not go slamming into the back wall. Or that allow you to throw a ball up into the air and catch it , even though the earth is moving through space in its orbit around the sun at around 66,000 MPH. The only thing that really matters here is acceleration - when the train first starts to accelerate the fly would indeed have to work hard to stay hover in one spot relative to the train.