*** Thanks for answers so far - I sort of knew much of it but wanted clarification - other perspectives help.
I suspected that overall friction may be the simple answer, but it still has the same effect on my head that the theory of flight does - I see and comprehend it well enough and can do the math to calculate it.... but the concept of how an aerofoil provides lift still raises an eyebrow when I look at an antonov or an A380

.
Anyway...
Large worms and successfu flywheel effect are counter intuitive given the discussion and the data I have from the report... the larger worm means much more friction via surface contact per revolution so it reduces efficiency .....so the large diameter flywheel effect is a net zero or even less... clearly a "Bachmann marketing thought".. not an engineering one.
One of the conclusions of the report I mentioned was that the worm diameter should be as small as possible and the pitch of the worm thread as large (coarse) as possible to reduce friction and improve efficiency... which means no greater than say 10: 1 ~ 15:1 is really practical in stage 1 (worm and first gear) for reasonable efficiency.
The ONLY loco I have ever seen that came close was an old cold war era PIKO - the worm was
fabricated via wire soldered straight to the motor shaft! It explains now why it actually ran very, very well... at the time I just considered it a primitive but clever answer to keeping cost low.
I wonder how practical turning the worm as part of the motor shaft design would be?
Interestingly most US diesels have a 12:1 ratio and are good at slow running and have excellent pulling power, but slow running high torque motors. 8 or ten driven axles is a good way of keeping power transfer acceptable of course... but its still food for thought
......... Hmmm
So a high speed box with a tiny coarse worm being the better compromise - perhaps we need to team them with either with a planetary box before the worm or added spur gear reduction between worm and axle.
Question: I wonder how the cost balance "bevel gear vs worm" looks when this is factored in? I suspect (OK, sort of know

)that in the end the margin between them is going to be very thin indeed.
I'm sure a few eyes are glazed over about now
But.. It matters to me.... as a loco builder who also nows loco buiders who would also love to be able to use tiny motors with prodigious performance via efficient drives... and... while its not a money spinner.... I'd personally love to produce the answer as a DCCconcepts product as I have the suppliers that can do it... but how many other modellers really care.... Enough to make it worth doing? I wonder....
Richard