O/O27 - 1:48, 1:64 ???

To add to what's been said above, keep in mind the history of Lionel: that the trains were once made and marketed as TOYS. Of course, father's thought the trains were cool too. But the target audience was KIDS! And most kids don't care as much about prototypical proportions as they do PLAY value and fun. It's about watching that 20 foot tall blue man jump out when the train goes by. And on the same layout, watching the 4 foot tall blue man toss milk cans out of a small reefer.
It has been theorized that the somewhat less-than-scale size of the 6464 box cars was intentional so they could be handled more easily by young boy's hands. Many of the oversized accessories that have been around for years were designed at a time when motors and light bulbs were not nearly as small as they are today. Again, there was also the idea of making the signals and gates visable from any part of a layout - in front or in back of a passing train.
ALL of us who played with trains in our youth were hardly concerned about any of this "scale" stuff. Those who don't believe me, let some young boys over to play with the trains and see what matters to them...
NOW things ARE DIFFERENT. The 3-rail train hobby is overwhelmingly an ADULT hobby... a visit to the YORK train fair will prove that. There is little question that the adult buyers are the ones driving the hobby now. And the concerns of adults are different from that of young boys. This may account for the more traditional and smaller sized items going to stater train sets. Whereas nearly all of the new tooling today is being aimed at the adult buyers who are demanding full-scale sized items. Of course, these newer scale sized items are very often priced accordingly.
There was an article in Classic Toy Trains in March 1993 issue that may get overlooked as this was the famous now-famous issue with Neil Young's layout. The article called "Scale's The Thing" could certainly be updated and reprinted. Basically the article broke down the various models of various makers trains by percentage of full scale. To my memory the catagories were full scale (100%), near scale (93-99%), medium (85-92%) and small (80-85% of full scale). I'm going by memory here: I might be off a little on the numbers here. I recall items like the Lionel GP-9, RS-3, NW-2 were full scale. Whereas things like the 4-4-2 steamer in many current Lionel starter sets rated under Small. Weaver Alco FA's were Scale, Lionel and K-Line Alco FA's were under Small. Of course, now Lionel has a new scale sized Alco FA.

So Dmestan, as a first step figure out how big your layout will be. The more room you have, the easier it will be to buy scale sized items. Consider what you can spend, since the newer scale sized items tend to be more expensive. Also bear in mind, a scale sized 18-inch long SD90-Mac looks as silly and toylike going around a 031 curve as does your Lionel 4-4-2 going around an 027 curve. All things are relative.
I myself have a small layout with 027 track, though I am without question a hi-railer as I add details to everything. I do have to play around with the "scale" ruler, but I still like detail and implied realism. I repaint alot as well as kitbash my own locomotives. I kitbash accessories to make them smaller. I design and build my own accessories and operating cars. My 12-inch long U-Boat is small by "scale" standards, but looks very much at home on my layout. As do my smaller Geeps and RS units. I basically stick to smaller items. I have a lot of the 9-inch cars by K-Line and Lionel (not currently made by them though). I add details to all of them to improve their looks. Next up I have the traditional 6464 sized box cars and flats, like Industrial Rail and K-Line Train-19. I have a few of the larger Railking and K-Line items, but I buy them with much thought. I've taken some of these items like the Railking PS-2 hopper and made them look a little smaller. Even before the recent change to Railking items being more scale sized, Railking was always a little bigger than similar items. Like the new K-Line scale welded steel side box car has a very modern look, so on my layout, I pretend it's a hi-cube box car.
Anyways, that's how I approach it. The fun thing about the hobby (even though it can be a little confusing) is that there is NO wrong or right way. Figure out what makes you happy and go with it.
There was a photo in one of the train mags of a scale sized SD-90 Mac rolling by the old stnadby Lionel Operating Gateman... it could have been captioned "True Scale meets Blue non-scale." Obviously it didn't bother the guy who made the layout. I do find it rather amusing that the guys who MUST have total scale accuracy in their locos and cars, still run them on layouts with all of the timeless favorites of distinctly non-scale Lionel operating accessories. Confusing? Yep, but still loads of FUN. The FUN should be rule #1.
brianel, Agent 027