Complex sentence: Tesla's grandiose ambitions to build a worldwide system were in line with the spirit of the Gilded Age, a period of big infrastructure projects: transcontinental railroads, telegraph systems, telephone systems, and submarine telephone cables.
Split into simpler sentences: Tesla's grandiose ambitions to build a worldwide system were in line with the spirit of the age. The Gilded Age was a period of big projects and particularly big infrastructure projects: transcontinental railroads, telegraph systems, telephone systems, submarine telephone cables.