No differentials on streetcars, same as trains.
http://www.whitmores.com/applications/rail-lubricants-services.htm
http://cprr.org/Museum/Rail_Wear_Johnson/index.html
http://www.lincolnindustrial.com/asp/rail/rail.asp
I don't think it applies to streetcars. They use a flow of water on the tight loops at the end of a line, which they wouldn't need if the rails were greased.