How do I accomplish diesel start-up/shut-down w/10 year old Tsunami decoder

You probably have a DSD decoder.  Here is a link to the manuals.

https://soundtraxx.com/manuals/dsd-150-dsx

There is a mute command, probably function 8 or 9.    But an engine start or shut down, I don't know.  

The plot thickens:  Notes say NCE 408 decoder, my NCE PowerCab says Manufacturer=141, Ver 064.  When using the “Use Program Track” on my NCE PowerCab, which decoder specs is it reading?

If you have both decoders, I don't know which one it will read.    You need to turn one off to read the other.   I have some installs like this and I put on-off switches on the underside of the loco so I can turn off either decoder to program the other.

I am guessing it is reading the DSD.    The D408SR would most likeliy be ver 35 or 36 in my experience.   I have not seen other versions of the 408.   

I just checked, I have a Soundtraxx, that I thought was a DSX decoder, and the manufacturer code comes up as 141 also.   The version is 18 according to decoderpro from JMRI.    CV 38 is set to 64.    So you are reading the soundtraxx decoder for those results.

to my knowledge SoundTraxx does not have any startup-shutdown options like the QSI does.

The QSI feature is very useful in our club setting, where you can shut down the decoder. Avoiding any accidental activation of an engine.

I seem to remember that those Soundtraxx decoders did a shutdown if you push emergency stop after coming to a stop