Yeah, the Unitah had some very tight curves. There's a photo out there (just can't find it now) showing a Unitah train with short cars interspersed between the longer cars just to get around some of the tight curves.
The Mt. Tamalpais & Muir Woods also had some very tight curves along with steep grades.
I'm comfortable with the 10" radius loop on my 35n2 railroad. The loop is not designed for the main part of operations. Besides I'm using very short equipment.
If you look hard enough you'll probably find a prototype for just about anything.
Nye, Inyo & Esmeralda Railroad
I'd recommend that you head back to Page 1 and have a look at Post #1 and the pdf link and my response at post #4.
Going tighter than then the "Minimum Recommended Radius" or first column can get you into trouble very quickly if you cannot lay your track to a very high quality standard. If you can stay bigger than the "Minimum Recommended Radius", then you should be able to have trouble-free operations without stuff binding up in curves,
If you decide to push the edge of the Minimum Radius envelope, you really have to know what the rules are before you try and bend them. To get anywhere near the second column or "Absolute Minimum Radius" EVERYTHING has to be "just right" and you have to be very good at measuring and tweaking both track and rolling stock to get good operation at that radius.
My advice is to stay with the Minimum Recommended Radius or bigger if you can.
Unanderra in oz
Howdy Jonah and welcome to Freerails. Minimum radius for a Bachmann outside frame 4-4-0? I would imagine 18" is about right. Before you get into measuring and bending flex track, I would spend a bit and buy a pack of Atlas (or similar) HO gauge 18" snap track. Maybe even a full circle of the stuff so you can at least watch something run while you consider what to do next. On my old layout, I was able to get the inside frame 4-4-0 to handle about 15" (I run Shays and Porters) but I had to modify the lead truck somewhat and she still had too much overhang.
Everybody has their own way to do things, this is just my way...and it may not be right anyway!
At any rate, have fun and run a train today.
Woodie's suggestion about buying some Atlas Snap Track is something really worth considering. You might even pickup some 15" radius just to see if your 4-4-0 will squeeze around it. Even though the equipment runs on HO gauge track you're still modeling in O scale. I had a small On30 layout and I could get the Bachmann passenger cars around a 15" radius but the overhang was terrible. I also had (still have) a Galloping Goose that I kept to 22" radius. It just looked and performed better.
Right now I'm working on a 35n2 layout and I do have a small loop with a 10" minimum radius. But I'm using 4-wheel HO mechanisms for the locos and very short 4-axel cars so I can get away with it. Also, the loop is not part of the 'main line' (right now the 'main line' is only 26" long) and will be treated more as a short branch line so the tight radius won't interfere with operations or future equipment acquired down the road.
Nye, Inyo & Esmeralda Railroad
Thank you all for the advice! I asked the question since I have seen that minimum radius sheet for the Bachmann On30 locomotives floating around and was curious if anyone on the forum had actually experience with the locomotive. Again, looking forward to posting a thread about my new layout soon!