I have a flat spot die-cast K-Line gondola that I modified 10 - 15 years ago. It really works only on a heavy car. And it's free.
I used my Moto-Tool to grind 2, well, flat spots on the wheels of one axle, being very careful to get them directly across from each other, as real RR wheels are pressed on to the axle (just like modern 3RO, etc., wheels) and do not spin on the shaft. If one is flattened, both are, since the cause (grabbing brakes, for example) stops both wheels. Flattened wheels are generally replaced when found by inspection (real RR term "slid flat").
The K-Line gon makes a nice, proportionate bam-bam-bam as it goes by. Did I mention free?