Tried the polarity reversal and it works now, but the bell doesn't.  One way bell works, the other way the horn works.  Is that normal?  I am using a Lionel 90W transformer.

I have two Williams engines: an SD-90 and a Hudson.  The Hudson runs okay on my MTH switches.  The SD-90 does not.  The SD-90 will stop just before leaving the switch unless it is running pretty fast (~8 volts or more) and "coasts" through the switch and the electrical connection makes on the track past the switch.  It seems like it could be a design of the reverser board?  When a slight power blip happens crossing the switch, it takes that to be a reverse button?  I have checked voltage in all the "live" sections of the switch and all read normal.

Williams recommends 90 watts for the two engines that I have:  SD90 and a Hudson, however one of their service guys says 80 watts minimum - I had a problem that turned out to be low wattage in a Lionel transformer. 

Sounds like your situation could be borderline volt/amps that fluctuate with line voltage into your home or load from other things on your layout; causing available watts to move up and down.  I have a Lionel 90 watt that works well with either of the Williams - actually I can run both of the Williams at the same time with no problem, and of course the ZW works well too.