Stan,
Your test was helpful, thanks so much.
The readings were erratic on one and NG on the other diode. We thought we had a good match out of some old parts but it didn't pan out. This past Tuesday we installed two new Zener diodes that were 5 1/2 volts. The unit ran great although a little slow. So now we are thinking that maybe the voltage has to be tweaked up a tad.
Now here is the next problem. While allowing the accessory to run for testing; the rubber tire split in two. Of coarse it's listed as unavailable on the Lionel website. They are still pushing this product out; but where are the parts? 
