Complex sentence: The G&CU renamed the locomotive "Pioneer" and used it in the construction of the G&CU until 1850, at which time the locomotive was loaned to the Chicago, Burlington and Quincy Railroad for work in and around Chicago.
Split into simpler sentences: The G&CU renamed the locomotive "Pioneer" and used it in the construction of the G&CU until 1850. At that time the locomotive was loaned to the Chicago, Burlington and Quincy Railroad for work in and around Chicago.