Investigators for the Union Pacific Railroad say they know what caused a train to derail and destroy the only laundromat in Webster City. U-P spokesman Mark Davis says they used computer simulations and mechanical inspections on the rail cars for their investigation into the September accident.They determined the cause was a “rigid truck.” Davis says the problem prevents the wheels on a rail car from turning freely which causes cars to bind up as they round a curve, leading to a derailment. The laundromat’s manager was trapped inside the wrecked building and spent several weeks in the hospital recovering from his injuries.