Union Pacific Corp. has asked customers to reduce their use of the railroad as it works out problems with traffic congestion and crew shortages, the Wall Street Journal said on Monday.

Union Pacific officials are asking a few customers to use truck shipments, limit new railroad business and examine ways to reduce the number of train crews needed for loading and unloading, the Journal cited a company spokesman as saying.

Problems with its service have plagued the company since late last year, the Journal said.

Union Pacific's biggest headaches are in the Los Angeles area and along its active freight route running from California to Texas, the story said.

The Journal said the Omaha, Nebraska-based company underestimated how many crew members would leave after changes in the federal retirement law for railroad workers, leaving the company with too few locomotive engineers and conductors.

At the same time, the railroad has seen a jump in demand for rail service recently in line with the recovery in the U.S. industrial economy coupled with a strong grain harvest.