can anyone tell me the name of the film with Sidney Poitie.?

He is a detective from a big city and is in a small town. It starts at the scene in the main station. He is waiting for the train and the sherrif arrests him cos someone has just been killed. He would have been lost untill they find out he is a cop himself. Then he helps the sherrif find the real murderer.

The film plays with prejudices. Must be from the 60s or 70s. So film fanatics feel challenged, can anyone tell me wht it's called!