What is this old movie?

I MUST KNOW! It's black and white, I thought it was Mickey Rooney but maybe not. Anyhow the scene is the protagonist is riding on a train (remember, black and white movie. So, old school train)... so he s on the train, I think he s some kind of inventor selling his "inventions". After rating off the chemicals used in whatever he s trying to sell he s overheard by.... I think it was the conductor. Protaganist asked to repeat the chemicals used.... conductor realize they inventor made dynamite or some kind of explosive and is then thrown off the train... I know it s not a lot of info but It s all I have. Good luck and thanks.