Gentlemen:
Just got back to work from lunch...
I did the test as prescribed, whereby I disconnected the transformer from the tracks and touched the leads to the GE 55 under low power: nothing. Same negative result with the second bulb. I figured it was useless, but I retrieved my 3rd and final bulb to give it a try (I had one #55 in the 6557, and two NOS spares). Wouldn't you know it...the third bulb works! I put the cab back on the 6557, hooked the transformer back up, and sent the trains around the Xmas oval a few times while delighting to light wafts of smoke from the 6557!
In short, the original bulb in the 6557 burned out--even though the filament looks ok--and one of my two backup bulbs was bad.
I ran to my cell phone to quickly CANCEL my best offer bid on a 6557 being offered on eBay; whew!
Thanks to all for advice and support...
All's well that ends well. It's an Xmas miracle!