Hunter Harrison's transition has been a 'complete disaster,' one shipper said.

CSX Corp (CSX - Get Report) CEO Hunter Harrison apologized to customers for service disruptions in an email Monday, blaming the slowdown on resistance from some employees, Reuters reported.

The number three U.S. railroad has lost business as 80% of CSX's shippers have experienced problems and 40% have switched to rival Norfolk Southern (NSC - Get Report) , Cowen analysts found.

Shippers have reported unpredictable or lengthier transit times and convoluted, inefficient railcar routing times. One shipper called the transition under Harrison, who took over in March, "a complete disaster" that has put the shipper's ability to serve customers in jeopardy.

Harrison said some of the company's employees have pushed back on his aggressive cost-cutting measures that have been part of an "extremely rapid" pace of change. Harrison vowed to handle the "internal personnel matters" that have led to service disruptions.

Harrison previously led turnarounds at Canadian Pacific Railway Ltd (CP - Get Report) and Canadian National Railway Co (CNI - Get Report) .

CSX stock traded down nearly 1.5% midday Tuesday.

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