Complex sentence: The Middletown and New Jersey Railroad is the only railway left in the City of Middletown and not only using their own trackage but using some of the old trackage of the Erie Railroad and the old O & W right of ways.
Split into simpler sentences: The Middletown and New Jersey Railroad is the only railway left in the City of Middletown. In addition to its own trackage it uses some of the old trackage of the Erie Railroad and the old O & W right of ways.