It depends on track centers and easements (spirals), among other things.
But for a loco of this size to operate well (assuming prototypical
articulation) requires at least 30" radius curves. To LOOK good requires
more like 40" plus radius curves.
Most of the mass-market models (Rivarossi, Lionel, P2K, Trix) all
articulate the rear engine too. This allows decent operation down to
about 22" radius, but they LOOK simply AWFUL doing it. In addition to
making the model more flexible, it does reduce the front overhang
considerably. these models look 'ok' on curves or 30" or so.
I once successfully ran a Riv. Big Boy on as 15" radius loop ... the
appearance reminded me more like a rotating turntable bridge than a
locomotive. Interesting, to be sure, but it LOOKED gosh awful!
Id use a minimum of 36 inch radius for this engine. If your model is
prototypical (rear engine fixed to the boiler frame, only front engine
pivots), you'll need upwards of 48 inch radius, and even that will be a
bit tight. In any case, you'll need plenty of trackside room to clear
BTW, the Bog Boy wasn't used on ore drags AFAIK, DM&IR used Yellowstones
(2-8-8-4), which as it happens were heavier and more powerful than the
Big Boy, but couldn't run as fast - but then they didn't need to. The BB
was designed to haul freight (and occasionally passenger trains) at
about 50MPH; I read somehere that one was clocked at over 80MPH.
I'm sure that someone will provide more data on BB operation, and
correct my errors. :-)
Well, I have a Riv. Big Boy that runs on 22" radius and it is more prototypical
than 18" radius curves.
So 22" is not too bad, but I would not suggest anything smaller.
30" would be good and 40" as the previous post suggests would be optimum.
BTW--The big boy has problems w/Number 4 switches, so a Number 6 or larger is
Number 10 switches are available too, but they are huge and expensive.
I run Number 6 switches and number 7.5 Curved turnouts (walthers).
PS--also run a challenger & 2-6-6-6
Literally, the answer is somewhere very close to 80 inches radius.
Practically it is probably somewhere around 42 inches.
The 80 inch figure represents approximately an HO scale 10 degree curve, which is
tight by prototype standards. The 42 inch number represents approximately a 19
degree curve which no 4-8-8-4 in North America could possibly negotiate. In
pair of EMD Geeps coupled can negotiate no less than a 19 degree curve.
So then a prototype 4-8-8-4 would require something near a 5,730 inch minimum
while a pair of Geeps only need a 3,636 inch minimum radius.
This represents something approaching the absolute minimum radius that the
could negotiate. The actual operating radius with a train in tow and at any speed
above a walk would be somewhat greater.
Lastly, I don't think that the UP's 4-8-8-4s spent much time pulling ore cars.
were better, more powerful engines for that sort of work. Take a look at some of
DM&IR machines if you want to see ore haulers in steam.
Time flies like an arrow
Fruit flies like a banana
The GN's N class 2-8+8-0 locos also spent a lot of time hauling ore, as
did the smaller M class 2-6+8-0's. The N's were big locos, but still not
a match for the impressive DM&IR Yellowstones.
The GN R2 was pretty much the equivalent of a DM&IR 2-8+8-4, but,
interestingly, the GN's R class was almost unused in the iron range.
There were one or two exceptions, but these were RARE! Other than that,
a couple R class, at the end of their careers, were used as stationary
steam boilers for thawing ore stuck in the cars.
None of the GN's articulateds survive, sadly.
Fortunately, three of the DM&IR Yellowstones are still with us. One at
Proctor, MN, one at Two Harbors, MN, and one indoors at the Lake
Superior Transportation Museum in Duluth. This last example is very
nicely restored, and animated by electric motors, thought not operable
The doubly articulated models don't look TOO bad on 30" curves. A
properly articulated model will indeed look awful on such a curve. Be
sure and allow adequate track separations and trackside clearance. One
solution is to just NOT look at it while it's traversing the curves!
Annoying sure, but perhaps watching it run on the straighter sections of
the layout will be compensation. We all have to make compromises!
I assume you already have a Big Boy. If not, and you want a coal drag, get a
Rivarossi H-8 Allegheny. Per Riv custom the rear engine is not fixed and it
will negotiate down to 18 inches, though with a lot of overhang.
If you have tight radii, one thing to do is to follow the two foot rule. It
will look better going around a curve if you are at least two feet away from
it, and maybe down at eye level, not looking god-like from on high.