The Bombardier R-142s have radial trucks that allow each axle to move up and down (suspension) as well as allowing them to twist (one wheel moves forwards and the other backwards), the ultimate effect being that the angle between the wheels and the tracks can be closer to the optimal 90 degrees for much longer on curves, reducing noise and wear. The Kawasaki R-142-As do not have this feature.
R-142 radial truck:
The cars also have many cosmetic differences both inside and out. A brief overview:
The electronic destination signs: On the R-142s the route number is given by an LED display (like the front route sign) whereas on the R-142-A the same display is LCD (like the rest of the side display).
R-142:
R-142A:
The interior electronic destination signs: The R-142s have smaller, darker displays that blank momentarily between messages. The R-142As have larger, brighter displays that are on continuously (no blank period between different messages).
The ceiling grab-bar: On the R-142 this is attached to the ceiling using large clamp-like brackets. On the R-142A the bar is welded to the supports seamlessly.
R-142:
R-142A:
Outside carbody: The most noticeable difference here is the "character line" running down the side of the car at about waist-level. On the R-142 this is a junction between two panels, one at a different level than the other. On the R-142A it is a raised strip of metal.
R-142: Look just above the red markings on the side of the car.
R-142A: Look just below the MTA label.
These are only a few, but they're significant enough to allow you to reliably tell the cars apart.
Dan