There are two ways.
1) Add a sound container to your favorite freight car bogey and use those on your freight cars.
2) Use a track with a track sound attached. There are some but that's really old and it doesn't always sound good.
My recommendation is to look at the dependencies used on a Jointed Rail boxcar with squeaky wheels and clunking. First to get the kuid for the bogey, and second to disassemble the config.txt and see how it's put together, and third to copy any parts needed to recreate this on your own.
= Open the bogey for edit in your file finder or whatever it's called on the Mac.
= Copy out the sound files themselves and save them to the desktop.
= Open the config.txt file in your text editor and copy exactly as it is the sound-container. This includes all the {} associated with it.
= Once you've got this, revert your JR boxcar.
= View the dependencies of one of your more commonly used boxcars or some other freight car.
= Get the bogey.
= Clone the bogey.
= Open up your clone for editing in your file manager. Change the Username - so it reads something else in Contant Manager instead of the original name so you know what it is.
= Add in the sound-container as it is in the JR boxcar.
= Copy in those sound files into the modified asset.
= Submit and pray all is good. If not recheck your work...
= Once this works, open up your favorite boxcar, other than the JR boxca, to edit the config.txt file.
= Replace any references for the bogey with your cloned one. You will need to change the bogey kuid not only in the bogey container, but also in the kuid-table.
= Save and close the folder/or just the config.txt file.
= Submit and pray all is good. If not, recheck your work.
= Test.