A tray carrier system having a tray loading position, a tray off-loading position, and four parallel rails extending from the tray loading position to the tray off-loading position. A tray carrier platform is disposed on each rail, where each tray carrier platform is configured to move along the rail on which it is disposed between the tray loading position and the tray off-loading position. Each tray carrier platform includes a carriage for engaging the rail, swing arms attached to the carriage, retaining pieces attached to the swing arms for selectively retaining and releasing a tray, and means for moving the swing arms between an open position and a closed position. The retaining pieces retain the tray at a tray level when the swing arms are in the closed position, and release the tray and are disposed entirely below the tray level when the swing arms are in the open position. Two of the tray carrier platforms form a first tray carrier unit, and the other two tray carrier platforms form a second tray carrier unit. The first tray carrier unit is capable of passing underneath a tray carried by the second tray carrier unit in the closed position when the first tray carrier unit is in the open position.