This post was created to be a "Top Question".
The following website has detailed info regarding Metrorail etiquette:
On the right-hand side of the page, there are 7 "top tips". Click on each "top tip" for more detailed info.
Basic etiquette --- When on the escalators, stand to the right so that others who prefer to walk will have a clear path to do so.
Metro prohibits eating or drinking not only on the trains but in the stations.