Automatic Train Stop did exist for steam locomotives, but afaik it only controlled the brakes, and was only able to perform a penalty stop if overspeed occurred at a restricting signal. PTC requires a finer-grained control than that, including being able to slow the train automatically approaching a speed restriction, without a penalty stop, if the engineer doesn't initiate adequate braking to do the job. That's probably not impossible to do, but it would have to be done using only the brakes which might become a penalty stop more often than not.