A fly in my brain soup

Please help me in solving my predicament; actually it may be quiet simple for some but for a dunce head like me it is a very difficult problem

you see one day while boarding in a local train (like the subway system) a house fly also got in, not that I noticed him (or was it her?). Flies tend to be very inconspicuous till they find something that interests them. As the train started moving the fly started buzzing around as is their wont. The train gathered speed and soon it was speeding about 80 Kph. the fly was buzzing around casually.
I wondered what speed the fly was traveling.
Was it 80 km Per Hour plus the speed it was flying when it was in the direction the train was traveling, why it did not crash into the bogie when it was traveling in the opposite direction?
Basically it was not touching any part of the train when it flew about.
And when it landed on some part of the bogie did it have to adjust its speed to compensate for the speed of the train?
Did it have to make extra effort to stay afloat? if so then it must be using tremendous amount of energy and it should have tired easily. But it was doing it very calmly and didn’t look fazed out or disoriented and was merrily buzzing.
It looked like for the fly the train was stationary.
I wracked my brains this way and that but just could not come up with a convincing explanation
Please help me as I am not able to sleep in the night, there seems to be a Fly in my brain mocking my ignorance.
Thanks in advance.