I'm well aware of the problems with the first (CC) Niagara. I believe they were solved in the recent Vision release. I mention the Niagara and Y6 to emphasize that some O gauge locos have been made with a split chassis and separate gearbox, so it's technically possible. The actual performance depends upon the execution: materials, tolerances, etc., and that's where the problems came in. Of course those examples are only available to those blessed with O72 curves.
The use of an idler gear can reduce the gear ratio-- or not. Northwest Short Line (NWSL) used to sell driveline parts and gearboxes for upgrades, custom builds, etc. The most appropriate gearbox size for O scale applications was "0.6 mod". This gearbox was sold in an idler and a non-idler configuration. The gear ratio on both was 23:1. The idler vs. non-idler choice was more about driveline angle (or making the driveline less visible so that it doesn't spoil the appearance of the model.)
There's a discussion on groups.io (originally Yahoo groups) called "Repower and Regear" specifically dedicated to stuff like this. The majority of the posts are about HO & N but the principles are the same. There are occasionally drivetrain discussions on the 2-Rail Scale Forum here at OGR, and on other 2-rail Forums around the Web. How many of you have explored outside the world of 3-rail O gauge? I have done so regularly for more than 20 years, and based on what I've read we're being short-changed. The fact that all of MTH's HO locos had the design I'm asking for from Job One, is because that audience wouldn't have accepted them otherwise. Learn, ask the hard questions, and demand better. Let's work together to improve the breed!
