Subway and Throughway

Subway noun - An electric railway operating below the surface of the ground (usually in a city).

Throughway and subway are semantically related In some cases you can use "Throughway" instead a noun "Subway".

Throughway noun - A broad highway designed for high-speed traffic.

Subway and throughway are semantically related in road topic. Sometimes you can use "Subway" instead a noun "Throughway", if it concerns topics such as path upon which travel occurs.