What are 'Pizza Cutters'?
The wheels of older N Scale locomotives and rolling stock had wheels with deep flanges.
With the advent of code 55 and code 40 rails, the NMRA changed the specification for the wheel profile, making the flange less deep.
The deep flange wheels, euphamistically called 'Pizza Cutters', have problems with code 55 and code 40 rails, specially in the turnouts.
Replacement wheel sets are available to convert old pieces of rollingstock to the new standards.
Old locomotives will need the flanges turning down.
NMRA S-4.2 Imperial Standards Scale Wheels