From what I understand the biggest issue is getting the fire to fill that huge firebox, which it hasn't been doing. This results in uneven heating and resulting problems, such as staybolt breakage and seam issues. I think Ed's thinking is if he only works the engine lightly with the firing issues damage will be limited. While it would be cool to see 4014 work hard on a heavy train, where is the motivation for UP to do this? She's essentially just a showboat. She probably performs adequately for the intended purpose in UP's eyes.