Tom, there is nothing inherently bad about the PWM output as long as the DC waveform peak is 15V or less. At 100% duty cycle we do not want more than 15V to the Gilbert motors. You have two different beasts by also having the original 3 pole universal motors in some engines. The good news is these are very robust motors so the only way to damage them is over-voltage or stalling the motor with high current draw through it.
If you are planning to run these old Gilbert universal motors at very low speed using the PCM controller the motors will get hot but I doubt they will be damaged. There are a lot of harmonics in the PCM waveform and all they do is generate heat in the motor windings. Put a Gilbert tender with an AirChime Whistle in it on the track and listen to how loud it sounds, that is the harmonics. At low speed the back EMF is low (low motor impedance) so the short term current can be high, heating the armature windings. At higher operating speeds motor heating is not an issue because the armature impedance increases with speed.
A Gilbert steam engine with smoke using the original universal motor can draw as much as 2.5A; average when running is between 1.5A and 2A. Dual motor Alco PA's will draw up to 4A. Add on some passenger cars and the amperage can be over 4A. That is why I recommend a minimum 5A breaker. Also make sure your track is divided into blocks so that more than one train will not be continuously operating through the same breaker.
My layout has 10A breakers on the transformer outputs to the track. But the transformer has other fast acting electronic protection for all the sensitive Legacy electronics in the engines. The added protection is to limit fault currents and voltage spikes. Neither of these would be concerns in the simple DC engines you are running.
