What is a Reverse Curve?

[Anonymous]

I hate vertical curves that lift wheels off the track. Especially with long wheelbase steam.

[jchnhtfd]

Good idea, MC... fits the generating equations, too!

[mudchicken]

Spirals on the main track, simple circular curves in the backtracks and switches.

On horizontal curves, you want at least one carlength of tangent between the curves so that you don't get your railcar trucks cocked in opposite directions leading to an unstable railcar that tips over or pops off the rail. (Does a number on the centerpin)

The dreaded yellow surfacing machines cannot transition from tangent to simple curve. They need to spiral to get from one to another as does any stringline solution in the real world.

Jamie: maybe get them to think in terms of rate-of-change in grade instead of radius. Then (maybe) the uninitiated might catch on.

[jchnhtfd]

Quite true, Chuck -- so far as I know all railroad curves are laid out with spirals (at least all the ones I measured). Verticals are actually not circular, but a parabola (don't ask). The idea is, as you say, to reduce rapid changes in lateral acceleration. Spirals also give you a place to run out your superelevation, same reasons.

Not sure what other roads practices were, but CV always had a tangent between the sections of a reverse curve. Might have been short, but it was there...

[ChuckCobleigh]

QUOTE: Originally posted by mudchicken



(2) you have vertical and horizontal curves on the railroad. In both cases, you had better have some tangent/ straight grade between the reverse curves or you have headaches with trans and the surfacing machinery. You now are beginning to see why switches in curves are such a big no-no.

[banghead][banghead][banghead]

MC, I assume that rail curves are laid out to some spiral curve at the approach to the transition to tangent track, much as the good highway engineers do with road curves. (Not all highway engineers, just the good ones.) This is supposed to smooth out the differences in lateral acceleration of the vehicle, which would make the train ride less like a roller coaster.

I'll stop now, before I start having Engineering Mechanics 202 flashbacks. So long ago.....

[mudchicken]

(1) The "Y" turnouts are Equilateral Turnouts (proper name) and have slightly different different curve geometry [ two compound curves as opposed to one] than a tangent geometry turnout.

(2) you have vertical and horizontal curves on the railroad. In both cases, you had better have some tangent/ straight grade between the reverse curves or you have headaches with trans and the surfacing machinery. You now are beginning to see why switches in curves are such a big no-no.

[banghead][banghead][banghead]

[nobullchitbids]

[black][/black

Reverse curve? Think of cutting a turnout into a piece of straight track, then bringing the siding back immediately to run parallel to the main. The "S" portion of the geometry is the "reverse" curve.

Gradual reverse curves can be used to allow a line to "flow" with specific topography and take advantage of things like balancing cuts and fills to achieve the least grade with the least cost, but the key word here is "gradual." I've never seen a model turnout easier than # 10, and that is about the minimum where a real railroad turnout would start. Needless to say, even a real turnout of so low a number, if leading into a reverse curve, probably would put all of the equipment on the ties in a hurry.

Real railroads don't like reverse curves either -- on the Union Pacific, the middle passing sidings running Ogden to Green River -- used in both directions as needed -- the reverse portion of the curve is minimized even further by using a trio of high-speed "Y" turnouts. So, if the area can accommodate a # 24 frog for a particular divergence, by using "Y" turnouts here, each leg in effect is angling at only a # 48 angle -- much more reliable!

The basic rule: Modeler's should avoid reverse curves where possible and opt to incorporate between them a length of straight equal to the longest piece of equipment operated. Yes, space does not always allow this, but in such situations, relocating the turnout, e.g. making it a curved turnout coming out of a curve, can eliminate the reversal. These are better arrangements, particularly if you run full-length passenger equipment or steam locomotives with large overhangs such as Northerns.

[ndbprr]

An S curve really doesn't relate to climbing hills. Generally it is used when the topography demands it. That being said engineers laying out grades attempt to minimize the grade as much as possible so the track will wander wherever it needs to to do that. I believe the big ten curves on the D&RGW do that climbing out of Denver up to Moffat tunnel. Definiteloy something to avoid on model railroads as not allowing a straight section between curves will almost always be a derailment headache.

[Tim Burton]

How does an S-curve/Reverse Curve relate to going up hills? I know how switch backs do it, but is the only difference a Reverse curve being less curves for a higher grade?

[CSSHEGEWISCH]

A reverse curve is an S-curve such as the ex-Alton line through downtown Lemont, IL. Horseshoe Curve is exactly what it says it is, based on its appearance on maps and aerial photos.

[Tim Burton]

I know what switch backs are, but what is a reverse curve?

Also, what would Horseshoe Curve outside of Altoona be?