At the laying farm eggs are collected on trays. These trays are of pulp or plastic. Pulp trays are used once so no contamination of eggs passed from egg to egg by using trays for different lots.
Different farmers use plastic trays more than once. In this way contamination can occur by passing trays from farm to packing station and from packing station to another farm. To prevent contamination trays coming from a farm have to be kept separately from clean trays. Plastic trays also have to be disinfected before going to another farm.
When the packing station is not located at the layer farm the eggs are transported to the station in trucks. These trucks pick up eggs from more than one farm. Therefore they can pass on contamination from one farm to another. Trucks must be cleaned before entering another yard. A truck can be equipped with a cleaning system. Especially the tires should be cleaned. A farmer can make sure that building a basin at the beginning of his yard disinfects the tires. The truck will then drive through this basin containing disinfectant.
The truck may not come near the house with laying hens. The storage room must be positioned in a way that the truck or the driver cannot infect the chickens. The driver must account for a well hygiene of himself and his truck.
Two links with video of farmpackers.