I've collected and used a fair bit of old Marx track in recent years, including O34 curve pieces and the corresponding "long straights" and O34 switches. Those items have been out of production for a long time, but there is still a lot of secondhand material out there at train shows etc. "O34" has the same lower profile as O27 track.
I find the O34 curves very useful on O27 layouts, because they form concentric circles for double track with O27 curves. The track spacing is just right for typical O27-type trains of not-too-large equipment.
SCARM author Mixy has added a Marx track library to the latest version of SCARM. He has offered to add the O34 track parts if he has more info on part numbers etc. I've suggested that the generic description of "Marx O34" might be more convenient than old part numbers.
I've created O34-equivalent curves on SCARM plans by specifying 15.8" radius, 45° arc on pieces of Atlas 3-rail flex track. I measure the Marx "long straights" as 11.2 inches long. The long straights are sometimes handy in combination with other typical O27 track pieces to create track configurations with a minimum of custom-cut pieces.
I think Marx was very clever to make the O34 curves because it would complement an existing O27 layout - just add another track around an existing oval.
It would be interesting to hear more information about when Marx made the O34 curves, and if it was offered in certain sets, perhaps more premium sets with larger equipment like the E7 units.
Attached photos show a couple of my temporary layouts which combined O27 and O34 curves for concentric double-track curves.
