Complex sentence: The railroad between Chicago and Aurora was originally constructed, by the Chicago, Burlington and Quincy Railroad (CB&Q) in 1864, which also operated the commuter service until it became part of the Burlington Northern Railroad in 1970.
Split into simpler sentences: The railroad between Chicago and Aurora was originally constructed by the Chicago, Burlington and Quincy Railroad (CB&Q) in 1864. The CB&Q operated the commuter service until the railroad became part of the Burlington Northern in 1970.