While I couldn't find my 1370, I also have a Tech 4 that came in that box o' junk I picked up when I moved in here (literally junk except for a few items, the Tech 4, an Atlas/Roco S2 in LV that I can repaint for Reading, an Athearn SW remotored with a Maxon can motor, and a few misc. bits I can use. Most of the rest is Life Like train set level, though I do have a box packed full of Tyco cars I promised to send someone on here and just haven;t gotten around to yet - it's been packed up ready to go for 2 1/2 years now..oops.
Anyway, hooking the Tech 4 to my newest and fanciest meter, which has dusl display (so I can do AC and DC volts at the same time, or volts and amps at the same time) plus various peak hold options, I found no spikes whatsoever. Either when flipping the reverse switch with the throttle wide open - peak was the same voltage it displayed before and after flipping the direction. There were no spikes when turning the main power on and off while the throttle was open - it just jumped to whatever the setting was, no higher.
What's insteresting is the frequency measurement. It's very high, a few KHz, with a few millivolt amplitude with the knob against the stop. As you crack the throttle, the frequency goes to around 60Hz - 59.9 on my meter. As you increase the throttle, the frequency of the AC component goes up, once again several KHz at full throttle.
The biggest problem is the output is so sloppy my scope could not trigger on anything. The signal looked more like noise than anything. Definitely now PWM or high voltage pulses in this model, the pulses were all low voltage, below the DC voltage and faded out at higher throttle speeds - I think it's a side effect of their circuit design that gives the KHz frequencies when the pulse amplitude is in the millivolts.
I see no reason why this particular model would cause any problems with DC or DCC locos provided the DCC decoder was of the type that can operate on DC as well (Soundtraxx decoders before Tsunami are a prime example of ones that are DCC only).
I knew this fancy test equipment I can't stop buying would come in handy some day.
I'm sure if I dig more I will find the 1370. I found the box. Empty, of course. I'm not so sure it matters, since my train workbench and electronics workbench will be combined, I can just test locos with a precicion lab power supply. Pure DC, no spikes or overshoot, very little ripple, and it can be current limited in case something goes wrong. I tested the Mantua 0-6-0T I got on ebay that way. Guy said it wouldn;t run, hence cheap price. Soon as I looked at it I saw the DCC decoder in the cab - likely configured with DC operation turned off. However, it ran fine off my lab power supply, DC is not disabled.