54" inside 72"

I wouldn't run anything on curves any tighter than the manufacturer recommends. It's not worth the risk in my opinion.

@CALNNC posted:

How well will a loco that says 72" minimum handle the 54's with the straight sections?

It won't. 72 minimum is greater than 54.

Worse, straight then curve over and over beats the loco to death. That would be jerky as can be. Do it right or don't do it.

Use flex track or the right curve.

@CALNNC posted:

If you have a 72" MTH track curves and wanted to add another track inside the curve, the 54 inch curve  with 3 inch straight sections between them seems to be common, just to maintain an appearance of parallel curved tracks as much as possible.  How well will a loco that says 72" minimum handle the 54's with the straight sections?

It will handle the straight sections just fine. 

I have occasionally slipped a single 54 curve into a track that is otherwise 72.  Some cars and locos will handle it OK, but many won't.  But I'm with the others above - probably better to just run smaller stuff on the inside track.   

Thanks for the info.  Looks like I'd better stay away from anything smaller than 72 for the 2 locos I have with that restriction.  Yes, the curve plus the straight did look like it would make quite the jerky operation, but you see it used on several layout sites.

You can lay a 1/4 circle of 072 inside of itself and keep a nice even track spacing for as many tracks as you like