Any ideas regarding wheel slip?

Today in a Geep 38, I could hear what sounded like a wheel slip. Not a pronounced pitch, but barely audible. And I could hear the automatic sander solenoid switch on too. Nothing was being over stressed. I was pulling the train on a 25 mph branch line with 300 amps. On a few grades where I had to throttle up to 400 amps, I could hear the faint high pitch and the solenoid kick in. Now, the wheel slip light never came on - the amp needle remained as stable as a pool table - and the speedometer didn't fluctuate. Is it possible for just one axle to over speed and slip the wheels while the other axles are all behaving just fine? Being it is late Friday, there was no one around to ask. Everyone had already skipped out for the weekend. I gotta take this same Geep when I come to work next. I can always cut out traction motors until I get the offending little beggar. But I would appreciate any insight here I can take back with me.