Why does the conventional wisdom (and almost everything published on the subject) say that the isolated portion of a reverse loop must be "longer than the longest train" if the only portion of a train entering the reversing section and electrically affected by the change in polarity is the locomotive?
Mike
mreagant Why does the conventional wisdom (and almost everything published on the subject) say that the isolated portion of a reverse loop must be "longer than the longest train" if the only portion of a train entering the reversing section and electrically affected by the change in polarity is the locomotive? Mike
Because it is the surest way to avoid problems either now or in the future. If the reversing section is longer than the longest train, then there will be no reverse polarity problems to short out your system.
You are correct in that the reversing section need only be long enough to accomodate the engine, but if you make the reversing section that short, you limit your abilty to have lighted passenger cars, lighted cabooses, longer consists, etc. Plus, if a buddy wants to bring over his train and run it on your layout, the reversing section better be long enough to deal with his power pick up cars.
Rich
Alton Junction
if the gaps in the rails are not properly insulated, there is the possiblity of a metal wheel of any car bridging the gap causing a short
What about putting a "dead" section of track at each end of the loop, just a bit longer than a six wheel truck? I realize this can cause flickering lights in passenger cars and stalling of marginal locos but on longer diesels with all wheel pickup this shouldn't be a major issue.
Modeling the Cleveland and Pittsburgh during the PennCentral era starting on the Cleveland lakefront and ending in Mingo junction
I've understood this to be a practical matter beyond electrical considerations. If the reversing section is shorter than your train, you will come back through the turnout or wye only to come in contact with the rear of the train. As long as you can avoid running into yourself, it shouldn't be a big problem.
Richard
[quote user="Train Modeler"]
I need to modify this to take into account your point on electrical isolation. I believe that part of your question goes to the polarity of the switch points and frog and just trying to avoid bridging the polarity change as you reverse through the wye or turnout. You only change the points after the entire train clears and that is when polarity changes.
There are a lot of misconceptions already in this thread.
But the bottom line is that it isn't that hard to make the reversing section longer than any train you will run, and there isn't any real benefit to keeping it short.
In the case where the reversing section is actually a loop, it's a no brainer, the train can't possibly be longer than the loop anyway. In other cases you sometimes need to be clever, and realize that the reversing section doesn't have to be where it intuitively looks like it is. You can include a longer stretch beyond the actual reversing section, as long as you make sure the entire thing is isolated.
Take the time to do it right at first. A lot of people who try to cheat on the general wisdom eventually discover why it is called wisdom.
Jeff But it's a dry heat!
ruderunner What about putting a "dead" section of track at each end of the loop, just a bit longer than a six wheel truck? I realize this can cause flickering lights in passenger cars and stalling of marginal locos but on longer diesels with all wheel pickup this shouldn't be a major issue.
Why design in a weakness before you even start? There is always a better way. This is a bad idea.
Thanks for all the helpful and informative replies so far. I have a couple of specific responses:
1. A "short" dead section seems like it would be a problem with DCC Sound locomotives. Most of mine stop, even momentarily, and restart the sound cycle if they encounter even a very small dead spot on the track.
2. In the particular circumstance I was working on it could be a big problem making the isolated section longer than 'the longest train. I'm talking about a 3-wye siding off a main line that serves one urban warehouse. The geometery creates what is basically an equilateral triangle and there are cases where a switcher can drop-off or pick-up enough cars to exceed the length of one leg of the triangle.
I'll be trying several options and will appreciate any additional thoughts.
Vail and Southwestern RR ... But the bottom line is that it isn't that hard to make the reversing section longer than any train you will run, and there isn't any real benefit to keeping it short. In the case where the reversing section is actually a loop, it's a no brainer, the train can't possibly be longer than the loop anyway. In other cases you sometimes need to be clever, and realize that the reversing section doesn't have to be where it intuitively looks like it is. You can include a longer stretch beyond the actual reversing section, as long as you make sure the entire thing is isolated. ...
...
That's not always as easy as it you make it sound, especially on layouts with reversing sections with multiple entrances and/or multiple reversing sections. Also, by extending the reversing section beyond the logical ends you sometimes increase the likelyhood of having trains trying to cross both boundaries at the same time.
That will not work with lighted passenger cars, unless the cars only pick up current from one truck or they pick up one rail from one truck and the other rail from the other truck. Even then it may or may not work depending on the spacing of the trucks, both on the cars and between the cars. If the spacing works out to where one truck is bridging one end of the dead section at the same time another truck is bridging the other end of the dead section, then it won't work.
Mike, for your first point, yes, that certainly is a problem. Practically, there is no way around it if the dead section solves all other problems for you, and only creates this one. It's up to you to live with the physical realities of wheel-base lengths commensurate with power pickup and your track system. I would not have the dead lengths of tracks, but for other longer locomotives, an inch and a half of dead track is no worse than the same thing at my dead frogs on my Fast Tracks #8 turnouts. Even my tiny SW8 from Walthers/P2K manages to cross those with nary a hitch.
Point 2 is a concern only in the event that a metal object bridges both gaps at a given point in your reversing system when the rails on opposite sides of the gap are not in concert electrically. Essentially, it becomes a crap shoot on a mixed train with cars of various lengths. If you were only ever going to turn a passenger train with X cars of Y lengths, you could probably work out the placement of the gaps and not have a short when the reverser changes the polarity at first contact at the head end/far gap. And that's only if the passenger cars are not picking up electricity. If they are, it introduces yet another confound into the reverser's problem. I think.
Crandell
mreagant ... 2. In the particular circumstance I was working on it could be a big problem making the isolated section longer than 'the longest train. I'm talking about a 3-wye siding off a main line that serves one urban warehouse. The geometery creates what is basically an equilateral triangle and there are cases where a switcher can drop-off or pick-up enough cars to exceed the length of one leg of the triangle. ...
Have you looked at not making one of the legs of the triangle the reversing section, but instead having it on the other side of one of the wyes.
mreagant In the particular circumstance I was working on it could be a big problem making the isolated section longer than 'the longest train. I'm talking about a 3-wye siding off a main line that serves one urban warehouse. The geometery creates what is basically an equilateral triangle and there are cases where a switcher can drop-off or pick-up enough cars to exceed the length of one leg of the triangle. Mike
In the particular circumstance I was working on it could be a big problem making the isolated section longer than 'the longest train. I'm talking about a 3-wye siding off a main line that serves one urban warehouse. The geometery creates what is basically an equilateral triangle and there are cases where a switcher can drop-off or pick-up enough cars to exceed the length of one leg of the triangle.
Mike,
How about posting a diagram of your track configuration so we can more easily visualize, and comment upon, what you are trying to do ?
The reversing section only needs to be longer than your longest locomotive or your longest commonly used consist of diesel engines if all your cars are equipped with plastic wheels.
The problem you run into otherwise is that if an engine or a car with metal wheels is bridging the gap between the reversing section and the regular track on one end of the reversing section at the same instant that a car with metal wheels is bridging the gap at the other end of the reverse section, it causes a short circuit.
CSX Robert Vail and Southwestern RR: ... But the bottom line is that it isn't that hard to make the reversing section longer than any train you will run, and there isn't any real benefit to keeping it short. In the case where the reversing section is actually a loop, it's a no brainer, the train can't possibly be longer than the loop anyway. In other cases you sometimes need to be clever, and realize that the reversing section doesn't have to be where it intuitively looks like it is. You can include a longer stretch beyond the actual reversing section, as long as you make sure the entire thing is isolated.. That's not always as easy as it you make it sound, especially on layouts with reversing sections with multiple entrances and/or multiple reversing sections. Also, by extending the reversing section beyond the logical ends you sometimes increase the likelyhood of having trains trying to cross both boundaries at the same time.
Vail and Southwestern RR: ... But the bottom line is that it isn't that hard to make the reversing section longer than any train you will run, and there isn't any real benefit to keeping it short. In the case where the reversing section is actually a loop, it's a no brainer, the train can't possibly be longer than the loop anyway. In other cases you sometimes need to be clever, and realize that the reversing section doesn't have to be where it intuitively looks like it is. You can include a longer stretch beyond the actual reversing section, as long as you make sure the entire thing is isolated..
In the case where the reversing section is actually a loop, it's a no brainer, the train can't possibly be longer than the loop anyway. In other cases you sometimes need to be clever, and realize that the reversing section doesn't have to be where it intuitively looks like it is. You can include a longer stretch beyond the actual reversing section, as long as you make sure the entire thing is isolated..
That's a good point. Just as making a reversing section as short as the longest engine is not always the most practical solution, neither is making a reversing section significantly longer than the longest train because it increases the possibility that two trains will be entering and/or exiting the reversing section at the same time.
Vail and Southwestern RR In the case where the reversing section is actually a loop, it's a no brainer, the train can't possibly be longer than the loop anyway. In other cases you sometimes need to be clever, and realize that the reversing section doesn't have to be where it intuitively looks like it is.
In the case where the reversing section is actually a loop, it's a no brainer, the train can't possibly be longer than the loop anyway. In other cases you sometimes need to be clever, and realize that the reversing section doesn't have to be where it intuitively looks like it is.
I try to adhere to the same terminology and call it a reversing section even if it is a loop. As Jeff points out, a true reversing loop needs to be at least as long as the longest train for purely practical reasons.
But not all track configurations with reverse polarity issues are loops; rather, they are reversing sections of track. In that case, the length of the reversing section needs to be determined based upon the type of equipment being run on the layout and the relative location and configuration of the reversing section within the overall layout.
A reversing section that is longer than the longest train may well have multiple entrances, and may not appear at first glance to be a reversing section at all. The classic example is the diagonal across an oval. It looks like the reversing section would be the diagonal, and you train length would be limited. But that doesn't have to be the entire reversing section. You can include some, or even all of one of the lobes of the oval, to increase the length of the section.
You don't want to overdo it, because you do want to decrease the likelihood that there will be trains crossing multiple boundaries at the same time.
Thanks to all you guys for the helpful discussion. I didn't post a drawing because I was not sure how to do it, and I think I learned what I needed to know from the various comments.
This forum is a great source for helpful information.