Using the train wheels as alternators !!!

I was at the railway station, and the trains were just passing by, and it struck me that 

>> I was at the railway station, watching the trains pass by, when it struck me: why not connect an alternator to the wheels? It seems like a great idea since the wheels. are already in motion, allowing them to generate electricity from the train's movement. 

!! I don't know, but some drawbacks I found are that the mechanical arrangement would be an issue, the capacity of the generation might not be sufficient, and many more.