CEO Hunter Harrison sent a letter to customers Monday apologizing for CSX Corp.’s service issues, blaming employee pushback to the changes he is trying to implement.

“The pace of change at CSX has been extremely rapid, and while most people at the company have embraced the new plan, unfortunately, a few have pushed back and continue to do so,” Harrison wrote. “This resistance to change has resulted in some service disruptions.”

These service disruptions generated numerous complaints in recent months,…