Main line and Upper crust

Main line noun - The principal route of a transportation system.

Upper crust and main line are semantically related in society topic. In some cases you can use "Upper crust" instead a noun phrase "Main line", when it comes to topics like high-class persons, upper class of people.

Upper crust noun - Individuals carefully selected as being the best of a class.

Main line and upper crust are semantically related in society topic. Sometimes you can use "Main line" instead a noun phrase "Upper crust".