I will also add my discounted nickel's worth.
Not all axle lenghts and thread widths are identical in size.
They vary from manufacturer and type of
truck
used(Andrews-Arch Bar-Bettendorf etc so there are a few exceptions and some model manufacturers have been known to install the wrong size.
In most cases passenger cars take 36". The exception is the Riv HWs advertised by trainworld in MR that take 33".

The proto sets are popular replacements and come in 33"/36" plastic axles with either ribbed or smooth backs.
Early time era=ribbed. Later=smooth.

The non ball bearing Intermountain, 100% metal(long thin pointy axle ends) are good replacements for
light weighted cars, flat-empty gondolas/hoppers because they add a half of ounce or so. In some cases you have to spread the truck end frames to get these sets in there.
Check the replacements to make sure they have the same profile ends as the stock set.

Athearn, Walthers, MDC, AccuRail do well with
proto/kadee because of these same profile ends.
The kadee wheels do have a black finish that wears off and transfers to the track.

As long as you are going to improve the rolling performance, you should also get"THE TOOL", MR Aug 02, page 91. Its a 60 degree reamer that trims out the truck journal.

You can get this by calling Reboxx, 1-800-935-2195, "THE TOOL" part #T100

This infor has been obtained from other modelers. All my cars come with metal sets.