As it turns out I should have read the old hand written notes I made on the Arnold decoder instructions that came with these Rivarossi units.  This decoder is 20 years old and can only use a two digit address for the locomotive.

Except when it cannot.

I use a loco’s road number as the address and the road numbers of these two are 4008 and 4010.  For this decoder I used just the last two digits of each so I set the addresses as 08 and 10.  Well the Arnold decoder likes the 10 but will not accept a leading zero as in 08.  So the address of 4008 is just 8.

Long story short- I will eventually change out the decoders in these two locos.

Made some additional test runs today with the DC locos and found four that would not run.  Opened them up and cranked the flywheel around a few times and they ran.  In general most were fairly noisy and will need cleaning and lubrication prior to being decoder equipped and placed into service.

I forgot how noisy the old Athearn Blue Box Specials were.

A short video of one of the test runs.  Three Bachmann Spectrum DDA-40 Centennials.  Six motors turning 48 wheels.  No problem climbing the 2% grade that will be on the finished layout.
I could not find any prototype photos of Union Pacific consisting multiple Centennials.  The photos I did find had them in a consist with one or more six axle road engines.