Complex sentence: Transport on a country-wide scale began in 1710 when the General Post Office started to operate mail coaches along the main routes between towns: private operators added to the routes, and an established road system was set up.
Split into simpler sentences: Transport on a country-wide scale began in 1710 with the introduction by the General Post Office of mail coaches on the main routes between towns. Private operators added to the routes, and an established road system was set up.