Believe excessive speed prior to the eccentric rods breaking was the difficulty encountered by #6315. The Burlington 2-10-4's were rebuilt in the mid-1930's for high speed service on the Chicago-Denver main line. Top speed was determined to be 50-55 mph. The 6315 was being pushed by 5632, and an exuberant engineer, to 60 mph, which exceeded the machinery's capability. I was a youthful observer (age 14) of these events, as I rode this fan trip.
It should be noted that 6315 was a very able locomotive. 10 months prior to this trip it was hauling 8000 ton coal trains between Herrin Jct and Centralia, IL (Sept-Dec 1958).
Don't know 2-10-2 #5511's machinery factor, but rode behind Burlington's fantrip favorite, O1a 2-8-2 #4960, at 60 mph. She has 64" drivers. She was never rebuilt for "high speed" service !