In an interesting coincidence, a good friend has just inherited several boxes of trains from his late grandfather and reached out to me about building a layout. He had some questions about this very topic... here's what I wrote back to him; hopefully it can be helpful to others as well.
Track for our 3-rail trains is referred to as “O” or “O27”. They BOTH have a 1 ¼” width between the outer rails, so you run the same types of trains on them. They differ in rail HEIGHT, width (with ties), and the radius options available to buy. Think of “O27” as just a NAME, rather than a measurement. Generally speaking:
“O27” profile track has a minimum 27" curve radius, but 42”, 54”, and 72” are also available. “O27” profile track is 7/16" from bottom of the ties to the top of rail. These days, the big manufacturers are mostly ignoring “O27” profile track.
“O” profile track has a minimum 31” curve radius, but 42”, 48”, 54”, 60”, 72”, 84”, and 96” (and probably more) are also available, in a variety of styles and from a variety of manufacturers. “O” profile track is 11/16" from bottom of the ties to the top of rail. “O” is the standard for today’s O-gauge trains.
Almost all locomotives and trains will run on both “O” profile track (taller rail height) and “O27” profile track (shorter rail height), provided they are able to negotiate whatever your smallest curve radius is. Switches and other specialty tracks can add complexity to the equation, depending on the type and age of the locomotives you’re running.
If you’ve got 9’x5’ to work with and you’re planning to run your granddad’s old trains, I’d go with Lionel’s O31 Fastrack or O36 Fastrack. It’s easy and bulletproof, and you won’t have a problem running older Lionel stuff on it. You can do a lot with a pingpong table!
Let me know if I can be helpful as you’re putting things together, and definitely get the locomotives serviced before you try to run them. I’ve got several spare transformers I’ll give you, so don’t make any investment there just yet.
