Spinning the wheels

It's hard, if not impossible, to turn the newer engines with the flywheels in them.

.....

Dennis

It has to do with the gearbox. Depending on the geometry of the worm, they can be self locking, or backdriveable.
Most Lionel engines of late have backdriveable gearboxes, and the wheels will roll down the track when pushed. MTH's gearing will lockup if you try and push the locomotive.

I suspect the backdriveable gears for the Legacy stuff was very good for MU configurations.  I think virtually all my Legacy stuff has that capability, but I can't think of another maker that has similar gearing.  That's not to say they don't exist...