A conveyor comprising: a rail (26, 27) having an array of permanent magnets embedded in and extending along the length of the rail (26, 27) to form a permanent-magnet stator creating a magnetic field; a tray supported on the rail (26, 27) and having a top article-supporting surface and having a series of commutated drive coils as a forcer coacting with the permanent-magnet stator to form a brushless linear dc motor to propel the tray along the rail (26, 27); a tray conveyor segment extending from a first end to a second end in a conveying direction and including: a pair of the rails (26, 27) closely spaced and separated by a slot and having tops (28, 29), each of the rails (26, 27) including such an array of permanent magnets creating a magnetic field across the slot; wherein the tray includes: a platform forming the top article-supporting surface and a bottom surface supported on the tops (28, 29) of the rails (26, 27); a blade extending downward from the bottom surface and in the conveying direction to ride in the slot and housing the series of commutated drive coils; a tray controller driving the drive coils to produce a traveling electromagnetic wave that interacts with the magnetic field to propel the tray in the conveying direction; wherein the slot widens at the tops (28, 29) of the rails (26, 27) and the tops (28, 29) of the rails (26, 27) are convexly curved. a rail (26, 27) having an array of permanent magnets embedded in and extending along the length of the rail (26, 27) to form a permanent-magnet stator creating a magnetic field; a tray supported on the rail (26, 27) and having a top article-supporting surface and having a series of commutated drive coils as a forcer coacting with the permanent-magnet stator to form a brushless linear dc motor to propel the tray along the rail (26, 27); a tray conveyor segment extending from a first end to a second end in a conveying direction and including: a pair of the rails (26, 27) closely spaced and separated by a slot and having tops (28, 29), each of the rails (26, 27) including such an array of permanent magnets creating a magnetic field across the slot; wherein the tray includes: a platform forming the top article-supporting surface and a bottom surface supported on the tops (28, 29) of the rails (26, 27); a blade extending downward from the bottom surface and in the conveying direction to ride in the slot and housing the series of commutated drive coils; a tray controller driving the drive coils to produce a traveling electromagnetic wave that interacts with the magnetic field to propel the tray in the conveying direction; wherein the slot widens at the tops (28, 29) of the rails (26, 27) and the tops (28, 29) of the rails (26, 27) are convexly curved. a pair of the rails (26, 27) closely spaced and separated by a slot and having tops (28, 29), each of the rails (26, 27) including such an array of permanent magnets creating a magnetic field across the slot; wherein the tray includes: a platform forming the top article-supporting surface and a bottom surface supported on the tops (28, 29) of the rails (26, 27); a blade extending downward from the bottom surface and in the conveying direction to ride in the slot and housing the series of commutated drive coils; a tray controller driving the drive coils to produce a traveling electromagnetic wave that interacts with the magnetic field to propel the tray in the conveying direction; wherein the slot widens at the tops (28, 29) of the rails (26, 27) and the tops (28, 29) of the rails (26, 27) are convexly curved. a platform forming the top article-supporting surface and a bottom surface supported on the tops (28, 29) of the rails (26, 27); a blade extending downward from the bottom surface and in the conveying direction to ride in the slot and housing the series of commutated drive coils; a tray controller driving the drive coils to produce a traveling electromagnetic wave that interacts with the magnetic field to propel the tray in the conveying direction; wherein the slot widens at the tops (28, 29) of the rails (26, 27) and the tops (28, 29) of the rails (26, 27) are convexly curved.