Having been away from model railroading for 50 years, I have been avidly reading forums and purchasing a few references to "catch up" on technology. I purchased "The Complete Atlas Wiring Book" to see if there was anything new available besides DCC. I am not interested in spending the DCC kind of money for a 4 x 8 layout. However, I am interested in having cab control for 2. This is simply for the pleasure of having 2 trains running at one time, with independence of speed and direction. The DIRECTION gives me a problem when I read the Atlas sections on Cab Control. During my time away from MR, I have spent time in amateur electronics and computer repair, so my understanding of electronics is not nil, but also not "expert". There may be technology out there that hasn't crossed my eyes. The Complete Atlas Wiring Book continually refers to the "common rail" throughout the book. It leaves the impression with me that it is connected with metal joiners at all joints and is in effect one solid piece of rail - except, of course, in a reverse loop. This concept gives me no trouble until the book starts discussing Cab Control for DC. I missed the point, if made, how the different cabs would have control of the direction of travel in the block selected for each cab. I have always understood that this was accomplished by reversing the polarity of the 2 rails. With a common rail on a layout I think all trains would be heading in the same direction, regardless of which cab was controlling the other rail. IF the latter is true, then the SPDT Atlas block switches wouldn't work would they? To have independent CabControl in each block would require DPDT switches selecting which one of 2 cab controls to secure the power setting (direction and speed), wouldn't it? If I'm wrong please enlighten me with a simple wiring diagram of how direction may be controlled with SPDT switches. Thanks in advance for your typically good guidance.
I can't give you the specific reasons why common rail works with cab control, but I can assure you that it does. Having not yet made the move to DCC, I have used the Atlas components for more than 20 years with two cabs. As long as the common rail is kept, well, common, you are fine following the directions in the various Atlas publications. The only exceptions, as you noted, are reversing sections: reverse loops, wyes, and turntables.
I remember being very puzzled by this same issue when I was first was learning wiring. George is right. It suffices to say: it just works. Yes, it is surprising, but in fact two distinct electrical circuits can share a part of their wiring. If you think about it, the issue is there even when the circuits are running in the same direction. How do the electrons know which power pack to go back to? The answer is, they can go to either one, in either direction, and it doesn't matter. All that matters is the electrical values on the non-common parts, and those will properly balance each other out, because they are regulated by the power packs. I've never actually done the following experiment, but I think it will work, and you could do it without doing any harm. (Someone please correct me if I'm wrong!!!!!) Hook up two power packs to the same track. Put a loco on the track. Raise the throttle on one pack; the loco will move. Now bring the other throttle up. If the packs are set to opposite directions, the loco will slow down, then reverse direction if you bring the second pack's voltage higher than the first's. If the packs are set to the same direction, the loco will just speed up.
Jagged Ben, Another book that you may purchase that may provide the answers, is "Easy Model Railroad Wiring" by Andy Sperandeo, published by Kalmbach. It's much more up to date than the Atlas book, and discusses common rail vs. non-common rail. The Atlas methods all work well, but I believe they make the assumption that common rail wiring is done. Have a look, I really enjoyed that book.
I've done Cab control for years. Have always insulated both rails. Although I have experience operating DCC layouts, and it's nice, Cab is not only less costly, but the old fashioned "hands on" is a lot of fun. Makes it seem more like those days of timetables and train orders. My kind of world! Boxcab E50
As a user of common rail cab control what you need to understand is the basics of electricity. A power source always must return to its own source. How common rail works is you MUST use two different transformers otherwise both cabs on common rail would be a short circuit. With two isolated transformers neither interfere with each other therefore it works.
Chartsmalm, I've been running dual-cab common-rail block control for years using one MRC Tech-II 2800. It is easy to set up and wire once you understand the concept. The concept is that each rail is continuous throughout your layout, and separate from the other rail. To show this, pick either rail of your layout diagram as your "Common" rail, and color that rail with highlight marker all around your diagram. That rail is now your common rail. The other part of the concept is that a single-pole double throw (SPDT) toggle switch selects which cab will power the hot rail of each block. You need to use a toggle with a center-off position that will "kill" a block if you want to park a train in it while running other trains with both cabs. NOTE - The common-rail concept will NOT work if you have reversing loops, because the common rail will swap sides within the loop. Here's how you set up a common-rail layout....... First, divide your layout into blocks, as I'm sure you already have done or diagrammed. Second, make three bus wires, one for the positive output of each cab, and one for the negative output of both cabs. I use 12-3 NM w/ground, with the black wire for one cab positive, the white wire for the other cab positive, and the bare ground wire for the common negative from both cabs. I use 12AWG to minimize resistance. Third, wire the bare common wire to the Common rail throughout the layout, at least once for every block, more if you have long blocks to reduce resistance. Fourth, wire the white cab wire to one outside pole of every toggle switch. Make sure you use the same outside pole for every toggle to prevent confusion when you running trains. Fifth, wire the black cab wire to the opposite outside pole of every toggle switch. Sixth, and last, wire the center pole of each toggle switch to the hot rail (not common rail) of each block. I assume that before you start this project, you will have already made a control panel with a schematic of your layout, and each toggle has been mounted in its proper place on the schematic. Good Luck, and SCREAM if you need HELP!!!!
Not necessarily true I have a reversing loop on my common rail layout although I may have cut the reversing section on both rails. I'll habe a look and let you know, here is how i wired my reversing section
Paul, that's sneaky...... mg: But then again, you ARE a Signals Guy and do this every day..... :shade: :teeth: :thumbs_up:
Chartsmalm and all tuned in, My answer will not qualify me to win a popularity contest. The short and simple answer: STAY AWAY FROM COMMON WIRE. Why? When you go to operate trains in the opposite direction you will notice a halting and hesitation and sometimes stalling on behalf of your motive power. Engines sitting on a siding have been seen crawling along until they foul the switch or develop very hot motors to the point of potentially self-welding the brushes. Damage may occur to the transformers in the form of overheating. How do I know? This is what happened to me on my original common wire/rail designed layout... utilizing the Atlas selectors and etc. When I switched over to DPDT those problems aforementioned disapeared but it still takes discipline to assure, you do not move a train on cab A into a block controlled by a DPDT set to cab B. What's happened since my original layout? I've built a number of layouts, of the common rail design, for friends and family at their insistence. All the layouts was inflicted with the same symptoms. All were later changed to DPDT and problem resolved. I have built models of this with some of the new cab controls available. I've seen the computer controlled side of transistor throttle burned out and/or the memory for momentum and speed control go up in smoke. Do others deny this happens? Absolutely, however those who have found a way to make it work have...done the following... left behind most of the Atlas Selectors and learned how to put in breakers or fuses, resistors and capacitors to protect the power sources...installed toggle switches either SPDT or DPDT...not usually mentioned in the Atlas hand books. This said, I'l leave you to your own well informed decision. You can contact me privately for any further discussion. The last time I opened this pandor's box on another website resulted in reams of responses, in the form of denials and arguments both pro and con. IE., One friend of mine lost his MRC Transitorized throttle when he tried to convert to the common...thing. Gosh, don't everyone hate me all at once. Thanks for the read.
You should note that if you actually do this experiment to pull out both power plugs of the transformers at once. Never ever pull out only one plug on the primary (high voltage side), because if you do, you will have 110 or 230 Volts on the pins of the plug. Take care Michael
On the Other Hand Remember that...in a theoretical way...everything is common wire. After all there is a positive and negative at the wall outlet and how that is applied to the rails is a cause for deliberation. (If you guys need any more help with this, just let me know! )
My layout is block control, but uses 2 rail wiring in each block. Lots easier to trouble shoot, and I recommend it. I am surprised no one has questioned your assumption that DCC might be too expensive for a 4 X 8 layout. Have you looked at the starter sets put out by many mfgs? You probably won't have too many locos to convert, and there would be some savings of wiring components in construction. Plus, you will have more fun running two trains w/o constantly switching the blocks to make them pass and meet each other. I once had a 5 X 9 HO and it was a full time job running the control panel to make two trains run - so much so that we were only watching the trains to make sure they didn't overrun the blocks.
I'm (still) using cab control. It's a small room and a small layout and you can't get much more than two operators in there anyway. I have all those critters that aren't receiver-friendly, lots of small power, and on the ATSF side, I keep adding weight to units where the receivers would fit anyway. I have a friends layout I'm working on installing DCC though, so I don't consider myself a fanatic to either cause. I went subminiature DPDT toggles from the get-go (Radio Shack) and didn't use common rail at all. I know it 'should' work, but it has been nice to know that OFF is OFF is OFF in a block. For the additional wire and work involved for the average cab-based small layout, I question if it's really better to make it 'simpler'. I have a another friend that used common-rail two-cab in an HO layout, and it works, so it's not like I don't believe it. But I won't use it. When you have one of those truly annoying shorts on a layout, it's wonderful to be able to drop a block out and KNOW there's no way in Halifax something wierd is crossing over somehow. Off is Off.
I'm not sure I follow. Do you mean that if you had two transformers with DC output connected to the track, and you unplugged one of them, you would have (in North America) live 110 volt AC on the unplugged plug if the other pack was still plugged in? I don't understand how this is possible. At the very least, the throttles would have to be up for this to happen, right? EDIT: Okay, granted, I'm no expert on what the average modell rr DC throttle actually has inside of it, but after some reading I still don't understand how the electronics would do what you say. Assuming said throttle has a bridge rectifier to convert AC to DC, well...that device, as far as I can tell, just won't convert DC back to AC in the opposite direction. So unless I misunderstood what you said, I don't see how it's possible.
Let's not confuse people. The term "common rail" refers to a particular wiring method for model railroads that can be distinguished from other model rr wiring methods, such as strict two-rail, three rail, DCC, and probably more I can't think of. The fact that all the power comes from the wall has nothing to do with the meaning of "common" when making these distinctions.
Well, let me the first to jump on you a little bit... I rather doubt that your problems were due to a flaw in the concept of common rail, but I won't dispute that in many situations it may be difficult if not impossible to avoid problems. Indeed, I could never say that anything you might put on your model pike would always be compatible with common rail. Personally, I never had any of the sort of problems you describe on my old layout that I had in high school, which used common rail and Atlas components. But then again, my layout only had four common rail blocks and two reversing sections, and I used the simplest MRC tech II throttles. Plus, come to think of it, I rarely ran two trains at a time anyway, and my problems with dirty track and poor contact would have outweighed anything else. It seems appropos to mention here that using the common rail concept does not necessitate actually having the common rail be all one connected piece of track. Indeed, that's bad practice on a layout of any size or complexity. Gapping both rails and using double pole switches allows you to power down blocks, and that helps in finding the source of electrical problems. You can still use a common bus to the switches at your control station, and in this sense you are still using the common rail practice, but you can still know that OFF is OFF is OFF. Gapping both rails is good practice with any wiring method including DCC.
Common Ground I thought common ground was one ground for the whole layout. What you have just described is my layout with dpdt switches, gapped both rails, and a single ground for each control panel. I have not considered my layout to be common ground, but I guess there are "degrees" of common ground. I thought common ground or common rail was the same thing. Further, one rail was solid through the whole layout or it was not common rail. Anyway, good discussion. :thumbs_up:
"Common Rail" and "Common Ground" are two entirely different things. Common Rail is a method of wiring the low voltage DC to the tracks whereby one rail is continuous around the layout (reversing sections excepted of course) and power is supplied to the other rail by one or the other of two variable DC power packs. The common rail is the reference point against which the other ("running") rail is measured. If the running rail is positive w/respect to the common rail, the loco goes forward, it it is negative w/respect to the common rail, the loco goes backward. The issue that some have mentioned is when a loco crosses the gap between blocks that are set to different cabs. This will connect powerpack A output to that of powerpack B. If they are set for the same direction and speed, there will be little or no voltage difference across the gap - no problem; if they are different directions and/or speeds, there will be a voltage difference across the gap and a short between the two blocks will result. That's why it is important to have your blocks set correctly ahead of your train when running with cab control. Common Ground, on the other hand is the AC safety ground on the center prong of your power pack and is designed to drain away any danngerous current that may result from an internal short in the power pack on the 110 VAC side. It has nothing to do with the low voltage DC side, which in most powerpacks is double insulated from the high voltage side. Elsewhere someone stated that pulling the plug on one power pack but not the other would result in exposed voltages on the pins of the plug just pulled. NOT SO! You can't feed DC into a powerpack output terminals and get AC out of the plug. I assume that the author was referring to back to back connected transformers (the electronic ones, not MRR power packs). In that case the AC input is converted to AC output at a different voltage. This works both ways. MRR power packs, OTOH, START with a voltage conversion transformer (110 VAC to maybe 14 VAC), but have rectifiers to change the low voltage AC to DC, and rheostats or electronic circuitry to provide variable low voltage output to control train speed. These are one-way devices, and anything else would be recognized in UL and other safety codes.
My interpretation would be that you are using common rail wiring. My reasoning would be: on your layout the diagram of the electrical circuits is the same as on a layout where there is a ungapped common rail. Whether the circuits actually join at the track (an actual common rail) or just under the control panel (a common bus wire) doesn't change the physics of why it works. That's why I consider it two variations of the same method. On the other hand, if you ran two bus wires per cab to your dpdt switches you would not be using common rail. You would have completely discreet electrical circuits. The circuit diagram would be the same as if you ran each train on it's own layout. Personally, I've never heard "common ground" used to refer to cab control. It strikes me as technically incorrect, since the common rail (or common bus wire) is not actually an electrical ground, but can end up being either positive or negative for any cab, depending on the cab's direction switch. (And also, as dustard points out, "common ground" means something else.) "Common bus wire" would be an appropriate way to distinguish your wiring method from the method in which there is actually a one piece common rail. Good discussion, I agree.
