Back in the 60's I used to hang out at the California Ave train station in Palo Alto on the Peninsula line and watch the commuter trains come and go. I remember seeing and hearing Fairbanks-Morse locomotives before they were replaced by GPs and SDs. As they accelerated from a dead stop, the FMs would rev up, pause, rev up, pause, rev up, etc. Someone told me they were shifting gears, which is exactly what it sounded like. This brings to mind several questions:

1. Were they in fact shifting gears?
2. If so, how exactly did that work?
3. Why didn't the newer locomotives need to do so?

Inquiring minds need to know, even if it's 45 years later. Thanks in advance.