A former Milwaukee Road employee I know who worked on Goose Island said that as recently as the 1960s trains would stretch from the Division Street yard all the way up to C&E Junction on Kingbury just south of Cortland. They were assembled for the transfer runs back to the Galewood Yard via the Bloomingdale Line. The transfer crews used to get annoyed at such super long trains and having to pull them back.
The longest trains I saw were in the early 1990s when AKZO Salt/International Salt received strings of carloads of salt in covered hoppers Typically some 20 cars would be pulled behind a Soo Line GP-9. GP-9s were used in place of MP-15s from what I recall, potentially due to their better pulling power for long trains. Frequently a white Soo Line caboose was also used for the backup moves. At that time there were several customer on Goose Island that still received freight cars including Waste Management, the team tracks at Hickory and Division Streets, Big Bay, International Salt, and National By-Products.
Another very long train movement took place in November of 1996 when the Ringling Bros. circus train was parked on Kingsbury between North and Halsted, stretching the entire length. There are pictures of this train elsewhere on this site. A Soo Line caboose was at the southern end near Halsted.