I am drawing up plans for my first HO layout and the tightest bend will be 36 inches in diameter. Will this look terribly unrealistic and / or will I encounter running problems? 2/3rds of the bend will be hidden inside a tunnel.
that depends…You have an 18" radius curve. General consensus for that is short wheelbase steam locomotives (6 drivers tops), and smaller diesels (4 wheel trucks). You are also limited to 50’ cars tops (in general). It won’t look TOO bad if you use smaller cars/locomotives, and you can usually get away with the larger locomotives (though not always).
Usually minimum radii is kept to 20-22" (40-44" diameter) or more, if possible.
[#ditto] and [#welcome] to the forums.
Two other considerations:
First, be SURE you’ll have some sort of access to the hidden portion. Especially if you stay with the 18-inch radius (that’s to the centerline, by the way), you’ll likely have derailments in there. Whether it’s a lift-off mountain, an open back, or access from underneath, sooner or later you will have to get in there. Cleaning might be an issue as well.
Second, if you’re using flex track, solder the joints that will be in the tunnel. That will help avoid some derailments.
All excellent reasons to avoid such a sharp curve, if you possibly can.
I agree. Eighteen inches is quite sharp in HO, even if you are using appropriate rolling stock and locomotives. In other words, despite your most patient and determined efforts to have all the rails perfectly in line and level, with nice 1 mm spaces between segments to allow for expansion due to changes in temperature or humidity, the chances are higher that you will get more derailments on 18" curves than if you had 24" curves, no matter where they are. Tempt fate even further by placing the arrangement in a tunnel or other poorly accessible area, and you can at least double the probability that you will regret the decision.
What it boils down to is this: if 18" is what it has to be for any number of compelling reasons, then for sure take the extra care to ensure that you can run stuff fowards and backwards through that trackage at faster speeds, and see what happens…prove the track, in other words. Once you are confident in what you have, scenic it. But, keep a cover or some other type of access to it handy so that “Darnit, anyway!!!” moments will be only that…a moment.
I have 18" curves on my logging road. I run some larger engines, but it is diffiocult. They are really fussy and I ma adjusting some track all the time. I do have three articulates, but it is a challenge. I wish I had 24"
ALSO - Have NO track that you can’t get at at least by removing the tunnel side or something. You will have to adjust that track.
18" radius is all I have on my HO scale layout. This was done due to space constraints. I don’t have the room for anything bigger. I run 40, 50 and 60’ cars with locomotives ranging from F7’s (49’ from pilot to rear door) to E units (70’ from pilot to rear door), and they don’t look too bad. One thing I always check when I get a new locomotive or a piece of rolling stock is it’s minimum radius. If it won’t take 18" and can’t be converted to take it, I don’t get it. This lets out most passenger equipment, but I don’t run any passenger trains anyway.
Thank you everyone for the thoughts and advice. I will have good access from below and from the open side at the back of my layout as I left myself 22" between the long straight side and the wall. Unfortunately, 36 " is all I have to play with at that point and I really feel I need the loop there to make the layout more interesting…both for now and in the long run.
Thanks again.
Is that 22" an aisle, or is it access? If it’s just access, and without seeing a diagram, could you let it “bulge” for a couple of feet, down to 18" or even 16" (depending on your girth); just enough to stretch the loop radius to 20" or 22"? That’s a trade-off I’d make in a New York minute!
One good way to tell is to just test your trains on that radius before you nail anything down. I was surprised to see how tight of a radius I could use for my gauge 1 trains.
underworld[:D][:D][:D][:D][:D]
I as well have 1 18" turn left (36") but with a little care and over weighing the front cars have pulled a 65 car drag. If using flex track see if you can go 19" it will help.
By the way welcome as well.
Cuda Ken
If you are planning to model a mainline railroad with locos or rolling stock longer than 60’ in length, your minimum radius should be about 36" no less.
If you do not have the space fpr 36" radius (not diameter), consider modeling a trolley line, a short line, a narrow gauge operation or converting to N scale 
Also think about what era you’re modelling. Like Jeffrey, I am somewhat space-limited, so most of my turns are 18-inch radius. I model the 1960’s, so my engines are short diesels and generally 40-foot rolling stock runs behind them. I have a set of 70-foot passenger cars which run OK, but they have truck-mounted couplers which are very forgiving.
More modern equipment tends to be larger, both in engine size and rolling stock length. Further back in time, the rolling stock stays about the same, but road power once again gets bigger, with large steam engines. A lot of these simply won’t run on 18-inch curves. I just got a 4-6-4 Hudson, and it will negotiate my 18-inch curves just fine. (Yeah, there was one problem, but it’s clearly a trackwork issue and not due to 18-inch curves. In fact, the curve is 22-inch at that point.)
18-inch curves are not going to cause derailments by themselves. With good trackwork and properly-gauged wheels, you should be able to run reliably on the curves as long as everything is designed to take them.