N Scale wheels have evolved over time. Just like everything else, there are many varieties to choose from.
There are two specifications to condsider when shopping for a new wheel set; axle length and flange type.
The third specification for N Scale wheels is tread width but this is more about appearance than performance so I won't try to adress that here.
Finally, all wheels, no matter what the axle length or flange type, should have the exact same gauge of 0.297 inches back to back per the NMRA Specification S4.2 Wheels, Standard Scale.
You can check this with your NMRA Standards Gauge to make sure your cars will perform correctly.
The manufacturers have never settled on a standard axle length for their trucks. This can make one wheel set fall out of the frame and another so tight it won't roll if you don't get the right size.
The following is a general list of known axle lengths:
Arnold-Rapido - 0.573 in
Athearn - 0.540 in
Atlas - 0.553 in
Bachmann - 0.560 in
BMLA - 0.540 in
Con-Cor - 0.563 in
Deluxe Innovations - 0.553 in
Exactrail - 0.540 in
Fox Valley Models- 0.540 in
Intermountain - 0.553 in
Kato - 0.563 in
Lima - 0.549 in
Micro-Trains - 0.540 in
Model Power - 0.567 in
Roundhouse - 0.549 in
Rivarossi - 0.549 in
Walthers - 0.545 in
The sizes listed are not meant to be inclusive of every item ever manufactured by a particular OEM but should serve as a general guide for finding the correct replacement wheel sets.
N Scale flanges have evolved over time to try to become more prototypical in appearance and closer to scale.
There is great debate as to how important it is and whether this has affected the performance of the scale.
Generally speaking the answer is, it just depends!
The NMRA also specifies this as 0.022 inches in the NMRA Specification S4.2 Wheels, Standard Scale but there has been a lot of deviations to this spec over the years.
Flanges come in three basic varieties; Standard, High Profile, and Low Profile.