My son built the simple device shown below according to instructions he found on the following website:
https://makezine.com/projects/sound-card-oscilloscope/
  

Together with the piece of free software referenced on the site and the input port (typically the blue port) of your computer sound card this simple device makes an effective and useful oscilloscope for DCC.

You need to adjust the potentiometer on the board as well as adjust your time base and amplitude, also turn triggering off.Tie the board into the layout either directly to your bus or use separate leads to test specific blocks of track. Either way the whole thing ties in at the point marked probe on the schematic.

The photo on the left shows an actual waveform from my layout with engines idling.
I found it useful for testing for weak signal strength.
The one pictured is my son's and is a two channel scope. the one on the layout is a single channel model.
Happy modeling