Frogs and Things

[Willy2]

What are "Movable Point Frogs"?
Another question: How does a railroad determine how fast trains can go across cross-overs? I have heard of cross-overs that can handle trains at 40 mph and I have heard of ones that can handle trains at 50 mph.
Thanks in advance!
Willy

[CShaveRR]

You almost need a diagram of a switch to explain this one fully.

A frog (as you may or may not know) is the part of the switch where one rail crosses the other (it's at the opposite end of the switch from the points, which are wjhat's thrown to determine trhe route). Normally, there are gaps in the track at this point to allow the flanges to cross the other rail. A movable-point frog closes the gap in the favored direction, reducing the wear and tear on the frog and decreasing the possibility of derailment.

Crossover speed. First of all, a crossover is a pair of switches enabling a train to go from one track to an adjacent parallel track...a train goes "through" a crossover, not across it. (The term for one track going across another at some angle is "crossing"; "diamond" is often a synonym for that, as in the diamonds at Rochelle.)
Having said that, crossover speed is determined by the angle of the frogs...the less sharp the curve to get from one track to the other, the faster the trains can move through the crossover. I suspect that your movable-point frogs are coming into play at crossovers designed for higher speeds.

On some of UP's newer high-speed crossovers, with the distance between the two tracks being increased, the switches could be a quarter-mile apart (or more!). It takes three switch machines per switch (six total for the crossover) to line the route--two at each set of points and one at each frog.

(Coincidentally, one of UP's high-speed crossovers is at a place called Frog Pond, Illinois. I wonder how many frogs get caught in the moveable frogs there!) Hope I didn't muddy the swamp too much for you.

[michaelstevens]

Willy,

Here check out LB Foster's web site -- http://www.lbfoster.com/railproducts/index.html -- they'll surely have something, for you.

Happy frog hunting !!

[Anonymous]

I better way to look at how the speed is determined is the length of the switch points. The longer the switch point the higher the speed for the crossover. The angle of the crossover is also less.

[mudchicken]

CShaveRR speaks the truth....

Speed of the turnout is determined by the lead curve and the frog angle. Length of switch points is somewhat irrelevant. (More like the spread at the heel of the switch points, heel block or hook plates) Once you get to a No. 24 turnout, you quit having a switch curve and a lead curve and have one uniform simple curve....

AH-SO Young Froghopper (Mr. Willy2), the whole issue comes down to the radius of the curve(s) in the turnout, of which the switch is a part. The bigger the radius, the faster the speed. Sharper curves mean slower speeds. Ed down there in Houston is not setting any speed records in his switching yard because his switches and yard curves have very short radii and speed creates havoc in a yard environment....

How much geometry or trigonometry have you been exposed to yet i nJr. High/Middle School? Frog angles are referred to in terms of a unit right triangle. (1) Ignore the hypoteneuse, (2) the short side is always one foot, (3) the long side in feet is always the frog number. If it's a number 10 frog, the long side of the frog is 10 feet. [ When you start to think all the math in school is a pain, please remember that some of us get to railroad because of it...somebody has to do the sterring on the railroad....]

On a moveable point frog, the 18" tip of the frog moves just like switch points and covers-up the flangeway gap in the frog...

As an example: From Santa Fe

Frog/ turnout size Frog Angle Equivilent Radius Speed Switch Points/Used
d-m-s
No. 6.5 8-47'-51" 397.85ft. 5-10mph 11.0 ft. YARD

No. 8 7-09'-10" 602.65ft. 10mph 11/16.5 YARD

No. 10 5-43"-29" 941.68ft. 15 mph 16.5 YARD/MAIN

No. 14 4-05'-27" 1845.65ft. 30 mph 16.5/20/22 MAIN

No. 20 2-51'-51" 3766.75 ft 40mph 30 MAIN

No. 24 2-23'-13" 5424.25 ft 50 mph 39 MAIN

If you have little or no straight track between reverse curves and switches, you had better slow down...Cars wil rock themselves off the track or the trucks won't be able to turn quickly enough to steady the load riding on them. If there is no tangent (straight track) between reverse curves, the trucks under a railcar get turned in opposite directions and look out! (The laws of physics are sure to bite'cha and gravity works!)


Radius?-Railroaders don't need no stinkin' radius! [:D]
Degree of Curvature and Chord Definition rules the rails!

-back to work....

Mudchicken
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Mudchicken just learned that the forum does not lend itself to tables[:o)]

[Mookie]

QUOTE: Originally posted by michaelstevens

Willy,

Here check out LB Foster's web site -- http://www.lbfoster.com/railproducts/index.html -- they'll surely have something, for you.

Happy frog hunting !!

I went there and printed off some more things to read tonite!

Thanx

Mook

[Willy2]

Thanks guys!
mudchicken: Unfortunately, we haven't learned much about geometry and trigonometry in school yet. (Isn't it terrible how schools are teaching less and less?) Even though we haven't learned much about it yet, I think I understand what you were saying. Basically, the bigger the radius the faster the speed. Thanks again!

Willy

[edblysard]

Willy,
radius has no bearing on real railroads, only in the model world.
Radius is defined as: a line segment that joins the center of a circle with any point on its circumference.
In other words, a radius of 20" means you have a 40" circle.

Degree of curvature helps define how many degrees away from a straight line the track goes.
The lesser the degree of change from a straight line, the faster the train can go.

Using radius mesurements to lay out a curve on your layout is fine, you have a limited space to work in, and a given starting point (center of the circle) to use.
Railroads have no real need to concern themselves in this manner.
Remember, one of the definitions of inerita is that any object (train) moving in a straight line, will always move in a straight line, unless acted upon by a outside force(curve)

The sharper the degree of curveature, be it crossover, switch or curve, (outside force)the more centrifigal force will come into play, as the object attempts to return it to its original straight path.
Too many degrees of curvature, and inerita will win, and the train falls over.

We dont go out in the yard, and stand in one spot, then measure and draw a arc or circle to lay track on.

We do stand beside one track, and measure how far the distance is between the two tracks we need to connect with each other, and how far the distance is between the two points we have to cross from one to the other.

Depending on how far it is between these two points determines how many degrees of curve needed to crossover.

The closer the two points are to each other, the sharper the curve, or higher degrees of curve.

If the two points are far apart, then we can use softer, or less degrees or curve.

And crossovers are not arcs, but compound curves, both ends curve, in opposite directions from each other, a big S if you will.

If both ends of the S have the same degree of curve, you can go faster, but if one end has to have a sharper curve(not a good idea, but in yards it happens) slow is the word.

I work in a yard designed in 1924, when the longest cars used were around 40' in length.
We have a crossover that has the ends so close together that, with the huge and longer tankcars of today, it is possible to have the trucks on two seperate tracks at the same time, both trucks still going straight.
We try and avoid this crossover when we can, but, when we have to use this crossover, we tip toe the big cars through.

Quit thinking in circles, start thinking in terms of straight lines, and degrees away from straight lines.

Stay Frosty,
Ed

[dknelson]

If you can track down an old Tru-Scale HO turnout you will see a movable point frog in action.
As to frog size and speed, a cousin of mind who has his degree in engineering sold me a book of railroad engineering analysis. This stuff is really complex! And that book dated back to the 1930s or before, too.
Dave Nelson

[CShaveRR]

I was once given a "rule of thumb" by an engineering friend: double the number of the frog to obtain the speed. So, for example, a #10 switch should be good for 20 m.p.h., and so on. There will, of course, be a tendency to round off speed limits...downward.

He went on to say that if you have an equilateral switch, the number is doubled yet again (but, of course, the restriction would apply to both routes).

Just for you, Sister Jen, an equilateral switch doesn't have a straight route. Both sides curve an equal amount away from each other (hence the name).

[Mookie]

QUOTE: Originally posted by CShaveRR

I was once given a "rule of thumb" by an engineering friend: double the number of the frog to obtain the speed. So, for example, a #10 switch should be good for 20 m.p.h., and so on. There will, of course, be a tendency to round off speed limits...downward.

He went on to say that if you have an equilateral switch, the number is doubled yet again (but, of course, the restriction would apply to both routes).

Just for you, Sister Jen, an equilateral switch doesn't have a straight route. Both sides curve an equal amount away from each other (hence the name).

Brother Carl - had to read it twice - very s-l-o-w-l-y, but it did sink in. Let's hope it just stays there!

SJ

[kenneo]

Back in November or December or thereabouts, MudChicken (aka dirtybird) and I got into a discussion about these things like equilaterals and turnout speeds. I don't remember the name of the thread, but I do remember having to explain in detail what I was trying very hard to say in general easy-to-understand English. Failed at both, I think.

Still having a bit of trouble at that.

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