I didn't realize they actually wanted the wheels to spin, if only a little bit.
Around 1976 or '77, we on the Santa Fe added ten new EMD GP40X experimental units to the roster, numbers 3800-3809. They had 3500 horsepower, and so were suitable for high speed intermodal trains, but they also had a new wheel slip system that allowed wheel creep. These were also our first locomotives that electronically disabled m-u sanding above 4 MPH. I was an Engineer out of San Bernardino, then and was on the Engineers' Extra Board, frequently working in Hill Pool freight service, between San Bernardino and Barstow over Cajon Pass The ascending grade begins right away when leaving and is about 25 miles of continuous 2.2% grade.
I remember well the first few trips I made on GP40X's. They were on intermodal trains to Barstow, as well as in the Valley pool on the moderate undulating and curving Third District between San Bernardino and Los Angeles. They impressed me as fast engines, great for regaining speed after leaving curves, and very fast on accelerating from a stop. They were used in 4-unit consists when new and being tried. Then, I caught a heavy freight train to Barstow, over Cajon Mountain. We made good speed, around 20 MPH on the long straight pull to Keenbrook, but then we got into curves. That's when my first experience with wheel creep began, and we lost 5 MPH. But we kept going. I knew that multiple unit sanding would not work, and we kept climbing and squealing at 14-15 MPH. I didn't feel any surging, which was my main concern. Once over the top, we went down the east side of the pass, which is 1.6 % with good dynamic brake, but not as good as the 6-motor equivalent, the SD45-2's. Once east of the narrows out at Oro Grande, they easily rolled the train right along at the allowable 55 MPH (because we had empty cars in the train).
So, it was obvious that they were good dry-rail locomotives. However, we had rain not long after the last trip I described. I brought a train in (another drag) from Los Angeles with 4 of the GP40Xs, and it changed crews on the main line at San Bernardino. I went over to the roundhouse and tied up, then drove up the pass, to Blue Cut, just below Cajon station. It was just drizzling a fine rain, and I could hear the engines coming out of Keenbrook -- coming, and coming, forever, it seemed. The wheel creep system was not working so well on wet rail, and the engines were squealing, but also, were unloading and loading up. I could hear it in the engine sound. Speed was around 5 MPH. Right in front of me, the train stalled at Blue cut, on a wayside flange oiler.
So turn things forward a year or two. We received a number of GP50 locomotives, numbered beginning right after the GP40Xs. They were the same horsepower, but had a refined version of the wheel creep system, and performed better on wet rail.
The Mechanical Department hated for Engineers to use sand, because the dust got into the traction motors and there's no denying that it was abrasive. So, EMD granted their wish. Some of the old Engineers who had started in the 1930s and early 1940s ran the multiple unit sanders all the way up the mountain on heavy trains, because that's what they learned to do on steam, and also had to do on FT's, with their primitive wheel slip systems.
This is one of those things that was not welcomed with a band and a parade by the Locomotive Engineers, but it became the norm, complaints faded away, and we learned a few new skills in mountain railroading because of it. And the result was that trains still ran, and traction motor maintenance cost was reduced.