That's interesting about the Lionel trademark because if you Google "Fat Boy speakers" you will come up with a product for motorcycle audio systems.
However after doing some research of my own I am persuaded that the actual defining characteristic of these speakers is the closer on the back referred to above, which must help deal with the problem of acoustic resonance.
The rectangular speaker pictured above is identical to the ones I replaced in my 3rd Rail diesel and GS4. I was wrong about the wattage of those speakers; it is 5 watts not 2 but that spec seems to make no difference in this context. The one in the Jawn Henry was a very flat unit mounted in a custom frame. It looked a lot like the Tsunami speakers found in HO models and I think was the one rated at 2 watts.
All the models I have worked on have Railsounds 4 systems and a new speaker alone won't cure some of the difficulties of that system. There is still something like static in the sound effects but I found that using the same Lionel Fat Boy as Laidoffsick did improved the low frequency/bass sounds and whistle effects and the static was not so pronounced.
There is an earlier thread that actually got me started on the speaker upgrade project and this makes clear that it is not only speaker type that matters but what system is driving it and how it is mounted:
