Proper radius for N Scale steam locomotives

ggburke Jan 19, 2014

  1. ggburke

    ggburke New Member

    What is the proper radius for N scale steam locomotives: Pacific, Hudson, Mikado and maybe the Challenger or Mallet. Also, if I'm creating a double main how do I adjust the curves for proper spacing.
  2. David K. Smith

    David K. Smith TrainBoard Supporter

    Simple rule for all curves: use the very largest you possibly can. Space curves approx. 1.25-1.5".
  3. BoxcabE50

    BoxcabE50 Staff Member TrainBoard Supporter

    Depends more upon the space available. Go as large in radius as possible.
  4. glakedylan

    glakedylan TrainBoard Member

    I no longer remember where I found the reference, but it was back in my HO modeling days. I think it would apply to any scale.
    nonetheless, I do not know if it is still a standard or not.
    minimum radius 2.5 X length of longest loco (and/or) piece of rolling stock
    better is 3 x that length
    anything more than that is excellent!
    [thus, a 6" locomotive or piece of rolling stock would have a minimum radius of 15"
    a better suited radius of 18"
    and an excellence being found over 18" as much as space would allow]
    is this familiar to anyone else? does it still serve its purpose as for giving a sense of what radius should be?
    always appreciate the information shared in this forum.
  5. robert3985

    robert3985 TrainBoard Member

    Okay, I've got the flu (again) so I'm killing time by putting together a mega-post while coughing my guts out and sipping hot green tea....

    The question is too simple. It can be divided into at least three questions (1) What is the minimum functional radius for these locomotives? (2) What is the best-looking radius for these locomotives? (3) What is the proper prototype radius (scaled down to N-scale) these locomotives could negotiate?

    Since it's late and I don't want to think about it too much, I'm going to assume you want to know what's the minimum radius these locomotives will negotiate. In light of that, I'm going to ignore the question (because each manufacturer's locomotive has it's own radius negotiating capabilities).

    Since model railroading is not just about functionality, but also about how things look, the answer to your question will be independent of the space you have. I'll just assume that you have room enough to accommodate good-looking, broad curves.

    Broad curves in N-scale are commonly accepted as being a radius of 18" or above. Any N-scale steamer will negotiate an 18" radius curve just fine and look pretty good doing it (minimal overhang). Your passenger cars and trains will also look better on 18" or larger radii.

    For myself, I chose to have a minimum "mainline" visible radius for looks of 24" since I run super-large steam and diesels along with a lot of passenger trains. On not-visible portions, my minimum "functional" radius is 18" because every engine and car I have will run on that radius without any problems whatsoever.

    As for the rest of the layout, meaning non-mainline curves, I limit them to an 18" minimum radius where mainline engines (Big Boys, Challengers, FEF's, E's, PA/PB's, Turbines, TTT 2-10-2's) would run prototypically. Industrial areas and areas where smaller engines would prototypically run (excluding huge mainline engines), I go with a 16" minimum radius and this is where the NW-2's, S-4's, RS-1's, Geeps, F's, FA/FB, 2-8-0's and MacArthurs would run..."run" meaning drop cars off, and pick cars up.

    These are MINIMUMS and I always attempt to have the largest radii I can stuff into my layout space and still retain operability and good cosmetics. Truthfully, I think I've only got one "tight" area on the whole layout so far which is in the Park City Yard throat with one small curve that's 16".

    For instance, in my Echo Yard, since this was inhabited by the largest steam, diesel and turbine power Union Pacific had, the minimum radii are 24", BUT many of the tracks are at much larger radii, some curves being over 100" in radius, such as in this photo of a U.P. Big Boy pulling a reefer manifest where the cars are traveling over a radius that's in excess of 150" (157 point something if I remember correctly).

    Photo (1): The Big Boy is sitting on the diverging stock rails of a #12 turnout which has an effective diverging track radius of 82". Cars in the distance are sitting on a 157" radius curve:

    Photo (2): Here's a higher shot of this area so you can see the actual turnouts and their arrangement:

    On the other end of Echo Yard, where the little Park City Branchline Yard is located, turnouts here are #6's, with an effective diverging track radius of 23", which is still considered "super-broad" for N-scale and every steam engine or diesel I have will negotiate these just fine.

    Photo (3): East End of Echo Yard with Park City Branch Yard on the right. #6 turnouts here and big "super engines" are not allowed (although they'll run okay). Only Geeps, F's, FA/FB's, 2-8-0's and MacArthurs (Mikados) are allowed access to this yard and the branchline trackage, which has an 18" minimum radius:

    U.P.'s preferred distance apart in Weber and Echo Canyons, where Big Boy lived, scales out to 1.5" exactly. So, wherever Big Boy and Challenger go on my layout, the track center-to-center distance is 1.5" or greater. In yards, and industrial sidings, I space 'em 1.25" so I can still get my fingers in between the cars without knocking them off the track. If you're modeling a specific prototype, or like the looks of a particular prototype railroad, find out what their center-to-center track spacing was. Like DKS says, it's usually around 1.25 to 1.5" if they ran big steam.

    Here's a photo on Echo Curve that illustrates that double-track mainlines were not rigidly at the same distance apart everywhere on a prototype. This is coming out of Echo into Echo Canyon where the mainline tracks are at different elevations and going east for a short distance where they'll come back together (1.5") and be the same height and grade for a few miles. The radii you're seeing here are approximately 36" on the nearest track and approximately 30" on the furthest, highest track. The reason I don't know what the radii are is because they're large, spiral easements. You'll note they're also supereleveated.

    Photo (4): Echo Curve just east of the Echo coaling tower:

    So, even though the answer is pretty simple "use the very largest you possibly can." there are instances where you can get by with functional radii rather than radii that look the best. Other times and locations, "photogenic" huge radius curves are great from a cosmetic design standpoint. The recommendation is to keep visible radii as large as you can, but who cares what they are if you can't see 'em as long as they function perfectly.

    Since the OP was asking about steam, here's an aerial photo of Echo with two Big Boys and a 3700 Class Challenger in the scene doing what this entire LDE was designed for.

    Photo (5): East mixed freight pulled by a Big Boy at the Fairbanks & Morse Co. coaling tower, and another Big Boy in the distance waiting its turn. A 3700 Class Challenger in helper service (running light) waits on the siding for the east bound COLA, while the Park City Local switches the Park City Yard:
    Last edited by a moderator: Jan 19, 2014
  6. retsignalmtr

    retsignalmtr TrainBoard Member

    A member of my club runs a Big Boy on our T-Trak layout which has curve radius of 11" and 12-3/8" and it makes it around the layout OK. But it looks strange on such a tight radius. My Kato RDC's have trouble on the 11" curve. Minimum radius on my home layout is 15", so they work good and look better on the curves. The largest steam loco I have is a 2-8-0. Use the largest radius you can fit on your layout.
  7. David K. Smith

    David K. Smith TrainBoard Supporter

    Once again, Robert presents a thesis on a topic when "use the broadest curves you can" would suffice. (A meme about "too much to read" comes to mind...) [big winkies]
  8. TVRR

    TVRR TrainBoard Member

    Just as a matter of curiosity I got a group of steamers out to try on 9.5" radius Unitrack curves. An Athearn Challenger, Proto 2-8-8-2, B'mann 4-8-2 N&W, MP 4-6-2, and a B'mann 2-10-2 all negotiated this small radius curve without any difficulties - no binding, squealing, or other odd behavior noted. Mind you they did look a little odd doing this. I use the largest practical curve size with proper lead in to ease them but sometimes there just isn't a lot of room, particularly in branch areas.
  9. DrMb

    DrMb TrainBoard Member

    If by branch, you mean branchlines, I'm curious as to how common it was for steam locomotives bigger than 2-8-2's and 4-6-2's to run on them regularly in real life.
  10. robert3985

    robert3985 TrainBoard Member

    Hehehehe...yup, but it killed a couple of hours for me!
    When I get rid of this damned flu, the posts will be much shorter and fewer! ;) So, please pray for my quick recovery! :teeth:

    Bob Gilmore
  11. RatonMan

    RatonMan TrainBoard Member

    Not likely!
  12. TVRR

    TVRR TrainBoard Member

    Yeah that's what I meant. I don't imagine the bigger stuff did , I just wanted to see if they could.
  13. Westfalen

    Westfalen TrainBoard Member

    Branch line meant different things to different railroads, N&W used 2-8-8-2's on its mine branches and Santa Fe regularly ran 4-8-4's on the Grand Canyon branch but both would have been the exception rather than the rule.

    The truth is that even the widest radius curves we can fit in N scale even with space like robert3985 would be approaching the limits acceptable on a prototype branch line. The best we can do is keep away from the ridiculous extremes like squeezing those Big Boys and Cab Forwards around 9" radius because as long as we do the manufacturers will make compromises to scale to allow us to do it.
  14. Railtunes

    Railtunes TrainBoard Member

    To simplify this discussion, I learned a couple of "Rules of Thumb" many decades ago that have served me extremely well in planning for reliable steam operations.

    1: The FUNCTIONAL minimum radius for steam is 10 times the rigid wheelbase. For a "simple" steamer, measure the driving wheel wheelbase and multiply by ten. For an articulated loco, measure one of the rigid wheelbases (front or real engine of a Mallet, for example) and multiply by ten. Since virtually all model steam engines have a degree of side play in the driver axles, this is a conservative rule that should eliminate any problems with binding - and derailments - on too-tight curves.

    2. The ESTHETIC and PRACTICAL radius should be such that the centerline of the engine never exceeds the inside rail of the curve. This also applies to all rolling stock. Steamers longer than, say, a typical Pacific 4-6-2 or Mikado 2-8-2 may be most effected by this dimension with the center line overhanging the inner rail at the center of the boiler. Articulated engines like Mallets, e.g., Challengers and Big Boys, will have excessive end swing at the smoke box end of the boiler and this should be the determining reference for choosing minimum radius. It should never exceed the outer rail.
    This dimension will also affect how much widening of track centerlines of parallel curves you will need. But the biggest reason to follow this guideline is that, if the center overhang or the end swing of all rolling stock stays within the limits of the rail gauge, you will virtually eliminate the possibility of "straightlining" on curves, as long as you don't place super short (ore cars) rolling stock next to maxi (89'-foot piggyback flats) with body mounted couplers on both.

    Also, I recommend you read the article on choosing minimum radius for rolling stock that you will find in the online magazine Model Railroad Hobby. This is the simplest, most easily comprehended - and practical - explanation I've found on dealing with this topic.

    - Paul Ingraham, Coordinator, AsiaNRail
  15. Jeepy84

    Jeepy84 TrainBoard Member

    The predecessor to my Buffalo & Pittsburgh ran 2-8-8-2 Mallets on their branchlines to avoid double heading or needing pushers, just like on the mainline. The B&O continued the practice when they bought them til dieselization.
  16. Railtunes

    Railtunes TrainBoard Member

    P.S. to message 14: That should be Model Railroad Hobbyist magazine, January 2009 issue.
  17. cuyama

    cuyama TrainBoard Member

    Those are handy rules-of-thumb, which were originally published in compact form on the web pages of the Layout Design SIG
    Curve radius rules of thumb

    By the way, the full name and correct link for the magazine that republished them is Model Railroad Hobbyist. That article was in the First Quarter 2009 issue.
  18. bremner

    bremner Staff Member

    Depends on the road and the loco. A real EMD F7 needed a minimum of a 274 foot radius per my original operators manual. That would be about 21.5 inches. The Unitah in Utah used to have mainline curves that their narrow gauge 2-6-6-2T's would roll through that were under a 100 foot radius.

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