Common rail, good idea, or mistake?

[jeffrey-wimberly]

A friend of mine is considering using the common rail idea for putting insulating gaps in his layout. In this practice, you cut a gap in one rail while leaving the other intact. I'm trying to talk him into using DPDT's and cutting both rails so that he can have complete control over the blocks, but, he doesn't seem to think I know what I'm talking about, even though he can see that it works on my layout flawlessly. For some reason, he thinks common rail is better. I used to think that the common rail setup was the way to go. Through the years, I've learned that it is a big mistake. I've tried to explain this to him, but he won't listen. The narrative below is what I tried to explain to him, word for word.

The system I use now is to cut insulating gaps in both rails and wire the block through a DPDT switch. It sounds complicated but is really very simple. I use dual cab control through a MRC 280. I run the wires from cab 1 to the top two terminals of the DPDT and the wires from cab 2 go to the bottom two terminals. The center terminals got to the block. With the flip of a switch, I can use either power pack to control a block and there's no common rail problem to worry about. Watch your polarity when your doing the wiring. I try to consistently use a wire of one color for the inside rail and one of another color for the outside rail.

Now how complicated is that?

[ARTHILL]

I am also a fan of dual gaps. It is not that hard to wire. When I went to DCC the dual gaps were useful in a couple of cases and not any harder to wire. I am not sure if there are any advantages, these days, to common rail wiring. Maybe someone can add some to this thread.

[BRAKIE]

I prefer common rail wiring..I use Atlas selectors for block control..

Your method is  good but,IMHO wiring overkill...The club's layout has over 2500 feet of track including yard,passenger terminals,engine service areas,industrial sidings etc and uses common rail wiring with a dispatchers CTC board for block control..It works flawlessly week after week.

I am a strong supporter of block wiring KISS method.Less wires and  DPDT's  lessens the chance of a electrical failure.

[fwright]

Whether you use common rail or not really doesn't matter in conventional DC.  Pluses for common rail:

- You can run a single common bus around the layout to tie feeders at chosen intervals from the common rail.  If you should switch to DCC, one of your power buses is already in place.

- you only have half as many wires going into your control panels

- single pole switches are cheaper.  Or you can use the second pole for other purposes such as indicator lights.

Minuses for common rail:

- if you switch to DCC, multiple power districts cannot share common rail.  Within a power district, common rail is OK.

- some folks lose track of which rail is common - this wiring mistake can be hard to track down.  But one can also easily mix up which rail is which at the control panel or the track when wiring both rails of a block.

- if power routing turnouts are used, you end up with some gaps in the common rail anyway.  But the bus and feeders system still works well.

my thoughts, your choices

Fred W

[nbrodar]

I use common rail.  I started with Atlas components, so I'm comfortable with it.   It's also less of a rats nest behind the panel.  With a little planning, I have no problem keeping track of the common and control rails.

The only issue with common rail, is that the power supplies for each throttle must be isolated from each other. 

Nick

[TomDiehl]

If you have more than one power pack: mistake. Packs need to be isolated from each other electrically, and that includes the rails. Regardless of whether you're using blocks that can select which power pack or if a power pack is assigned to a certain set of blocks, any time they're not isolated, you stand the chance of feed through from one pack to the other. If the packs are set to opposite polarity, you stand the chance of getting double the voltage on the rails. Not a good situation for your motors or lamps.

[nbrodar]

If you have more than one power pack: mistake. Packs need to be isolated from each other electrically, and that includes the rails. Regardless of whether you're using blocks that can select which power pack or if a power pack is assigned to a certain set of blocks, any time they're not isolated, you stand the chance of feed through from one pack to the other. If the packs are set to opposite polarity, you stand the chance of getting double the voltage on the rails. Not a good situation for your motors or lamps.

That's not true..  As long as the power packs do not share are common AC connection, they are isolated.  I've used common rail for 20 years, and have never encourtered the situation you discribe.  Furthermore common rail wiring dates back at least to the 1950s.   If the system had such a glaring error, I doubt it would still be practiced after half a century.

The key is that the packs must not share an AC connection.  Each throttle must have it's own AC power source.

Nick

[Tom Curtin]

I have always used (for decades) common rail myself, with good results.  I do understand there are some block signalling systems that require both rails be gapped,

[rrinker]

 If it's a simple layout, it doesn't matter.

 If it is a more complex layout, and is just building, do NOT go common rail. On a complex layout you'd need more than one feeder to the common rail anyway. By not using common rail, you make switchign to DCC easier. Again the simple layout vs the complex - on a simple layout with just one booster, it really doesn't matter one bit if it's common rail or not. On a more complex layout where you might have 2 or more boosters, and/or circuit breakers for power districts - they all MUST be double-gapped.

 Common rail for signalling is not the same thing. Each DCC power district needs to be double gapped from the next one. But within that power district, most track detectors only need one side borken - so within the district one rail is continuous and the other is gapped to provide the blocks. But the whole layout is NOT common rail in that case.

                                                       --Randy

[cwclark]

common rail wiring is great for small simple layouts....however, if he plans on putting in electronic devises such as train detection circuits and the like, common rail becomes a headache...there are some electronic devises out there that will work with common rail but the majority of them won't...I found out the hard way because years agoon a previous layout, it was wired common rail and when i put in linn westcott's version of the "Twin-T" train detection circuit the whole thing went haywire...I had to go thru the whole layout and cut gaps in the opposite rail and rewire the entire layout so that the Twin T's would operate the ABS system properly....chuck 

[Don Gibson]

Nick, Randy, and Chuck are correct -

TWO DC power packs can share a common ground and use SPDT block toggles - as long as they have indepenent AC transformers.

DCC power boosters apparently cannot since they do away with block toggles.

SIGNALLING SYSTEMS - using resistance for detection - use COMMON RAIL wiring with BOTH rails gapped for block signalling.

They also detect on DCC. Don't ask.

[TomDiehl]

If you have more than one power pack: mistake. Packs need to be isolated from each other electrically, and that includes the rails. Regardless of whether you're using blocks that can select which power pack or if a power pack is assigned to a certain set of blocks, any time they're not isolated, you stand the chance of feed through from one pack to the other. If the packs are set to opposite polarity, you stand the chance of getting double the voltage on the rails. Not a good situation for your motors or lamps.

That's not true..  As long as the power packs do not share are common AC connection, they are isolated.  I've used common rail for 20 years, and have never encourtered the situation you discribe.  Furthermore common rail wiring dates back at least to the 1950s.   If the system had such a glaring error, I doubt it would still be practiced after half a century.

The key is that the packs must not share an AC connection.  Each throttle must have it's own AC power source.

Nick

Not quite. If one power pack has the polarity (forward-reverse) switch set to have the negative on the common rail and the other has the positive on the common rail, they can be putting 24 volts across the rails, or will result in a short. All it takes is one insulated railjoiner going bad. With both rails insulated, you'd need two joiners to fail.

And trust me, this will drive you NUTS to find the problem when this happens.

[jamnest]

My best arguments against DC common rail wiring is:

1. By having totally electicaly isolated bolcks you are already wired for DCC should you decide to make the switch at some future date.

2. Electrically isolated blocks make detecting problems easier.

Prior to converting to DCC I used two MRC-20 walkaround power packs for my layout.  I used DPDT/center off switches for the layout.  I electrically isolated each block.

Hey, it worked for me.

JIM

[fwright]

Not quite. If one power pack has the polarity (forward-reverse) switch set to have the negative on the common rail and the other has the positive on the common rail, they can be putting 24 volts across the rails, or will result in a short. All it takes is one insulated railjoiner going bad. With both rails insulated, you'd need two joiners to fail.

And trust me, this will drive you NUTS to find the problem when this happens.

So you are saying that common rail should be abandoned because of the chance that an insulated rail joiner will go bad AND the adjacent block is set to the other power pack AND has its direction toggle set to the opposite polarity . 

First of all, any respectable power packs are going to shut down immediately from the short circuit to protect themselves.  Secondly, this fault is readily identifiable from the need for opposing polarities from the two power packs.  My last argument is that failure of an insulated rail joiner or a gap with a piece of styrene glued in it is a lot rarer than my chances of crossing the 2 wires going to a block compared to the rest of the blocks.  I personally can't see dealing with all the extra wiring at a block control panel to protect against a failure of a gap.  But then these are

my thoughts, your choices

Fred W

PS  A properly wired common rail DC block control layout is no more difficult to convert to DCC than one that is double-gapped.  In the common rail case, a second gap will have to be added where the power district boundaries are.  How many layouts require more than 4 power districts?  Most home DCC layouts can get by with one power district quite easily (the number of power districts being a function of the current required to run the number of trains running simultaneously on the layout), especially with the block toggles for fault isolation already in place.  So the number of gaps to be added is very few.  A common rail setup of any size (I did it on 4x6 ft layouts) is going to run a bus wire with feeder drops to the common rail to avoid voltage drops and loss of continuity at rail joiners.  So one of your power bus wires for DCC is already run and in place.  Simply add more feeders if needed.  That's a lot easier than tapping into the home run wires to the block control panel to add feeders.

[rrinker]

  This is why I differentiated between simple layouts and complex ones.

On a complex layout, if you have home runs and double gaps with DPDT toggles, it's super easy to convert to DCC even if you use two or more boosters - just connect each booster to a subset of the block feeders and since you already have double gaps between every block - no issues. You could even leave the toggles in the circuit, connect booster A to toggles 1-9, booster B to toggles 10-19, and boosert C to toggles 20-29.  If you had common rail you's aldo have to go and cut additional gaps and possibly add feeders to the former common side. Much easier if it never had common rail wiring to begin with.


Which detection system needs double gaps in each detection section? Diode-drop detectors like the BDL-168 do not, nor to transformer types like the DCCOD and Rob Paisley's circuit.

                                           --Randy

[Don Gibson]

... If one power pack has the polarity (forward-reverse) switch set to have the negative on the common rail and the other has the positive on the common rail, they can be putting 24 volts across the rails...

Tom:

Isn't the idead of 'common rail' to have NEGATIVES tied together, and the "Other" (+) connected through SPDT  switches?  That puts the power packs in (potenial) Parallel if from block to block, and the engine can get (+) and (-) at the same time, and yes, popping crow bars, but isn't that  the same with dual gaps and DC? It is on my RR.

[TomDiehl]

Not quite. If one power pack has the polarity (forward-reverse) switch set to have the negative on the common rail and the other has the positive on the common rail, they can be putting 24 volts across the rails, or will result in a short. All it takes is one insulated railjoiner going bad. With both rails insulated, you'd need two joiners to fail.

And trust me, this will drive you NUTS to find the problem when this happens.

So you are saying that common rail should be abandoned because of the chance that an insulated rail joiner will go bad AND the adjacent block is set to the other power pack AND has its direction toggle set to the opposite polarity . 

First of all, any respectable power packs are going to shut down immediately from the short circuit to protect themselves.  Secondly, this fault is readily identifiable from the need for opposing polarities from the two power packs.  My last argument is that failure of an insulated rail joiner or a gap with a piece of styrene glued in it is a lot rarer than my chances of crossing the 2 wires going to a block compared to the rest of the blocks.  I personally can't see dealing with all the extra wiring at a block control panel to protect against a failure of a gap.  But then these are my thoughts, your choices

Fred W

Yes, that's what I'm saying. I can't see making more potential problems for yourself, but that might be based on my 33 years as an electronics technician.

BTW, with common rail, only one insulated railjoiner needs to go bad or get something conductive across it (don't get me started on the people that just cut a slot in the rail for "air insulation"). The other "and's" will happen, and trust me on this, intermittent problems are a real SOB to find, especially if you have a hard time determining what the conditions are that cause the failure.

On a large layout, you're going to be dropping feeders from both of the rails at regular intervals, whether you're working with common rail or not, so you're not saving anything on wire with that in mind.

[BRAKIE]

Don,Atlas selector uses  common rail and you can use 2 power packs connected to the selectors without any problems.This is the method I have use for years.

[gmcrail]

Both methods work well when correctly wired.  I have used common-rail wiring for more than 50 years, with good results.  I prefer it for three reasons: 1) It minimizes wiring to and from the control panel; 2) SPDT switches are cheaper than DPDT; and 3) you don't have to have gaps in both rails at the same location.  Another advantage is that it works better with Priority Cab wiring, which is the only system I know which allows you to have more than two cabs on a layout without using (expensive and bulky) rotary switches.  All this in DC, of course.   

[D&HRR]

DPDT switches are the way to go for more control with DC style layout. Thats what I use on my layout and I can run many different trains in many different directions with serperate power packs.

[TomDiehl]

... If one power pack has the polarity (forward-reverse) switch set to have the negative on the common rail and the other has the positive on the common rail, they can be putting 24 volts across the rails...

Tom:

Isn't the idead of 'common rail' to have NEGATIVES tied together, and the "Other" (+) connected through SPDT  switches?  That puts the power packs in (potenial) Parallel if from block to block, and the engine can get (+) and (-) at the same time, and yes, popping crow bars, but isn't that  the same with dual gaps and DC? It is on my RR.

With a polarity reversing switch (forward-reverse), the same wire isn't always negative.

[jeffrey-wimberly]

I found out a long time ago, when I was using common rail on a layout with a common rail connected throughout, that there was no way I could have two trains running in oppisite directions on the same track, even with separate power packs, because they short each other out.

 

I switched to dual rial block control and can now run two trains in opposite directions on the same track with two separate packs, using DPDT switches to control the blocks.

 

In common rail, one rail is intact all the way through. The other rail is gapped with power being supplied through a SPST switch. Try running two trains (DC) at the same time in oppisite directions on this track. It can't be done, unless the motor leads of one locomotive have been reversed. This is the reason why reversing loops require DPDT switches. When positive comes in contact with negative, you get a short.

[tomikawaTT]

If common rail is a mistake, then I have been happily mistaken for over half a century.

The straight DC control system I'm currently using is a common rail adaptation of Ed Ravenscroft's MZL scheme.  Using common rail allows me to reduce the track power wiring at the panel by approximately 45%, not to mention the savings in (electrical) switches with half as many poles.  Over the years, just the latter has freed up enough money to buy several brass locomotives.

The key is, there is no direct connection between locomotive power supplies downstream of the 120VAC house power connection until the controlled-voltage DC passes through the DPDT reversing switch at the locomotive controller.  With even the least amount of care, no train will ever bridge a gap with a 24VDC differential.  They only occur where two zones meet, on the connecting track halfway between 'towns,' and there are 24V warning bulbs masquerading as house lights at those points.

One thing which common rail allows is that detection blocks don't have to correspond with control sections, either in length or in location of boundaries.  I do gap the common rail, both for detection and to ease troubleshooting.  If I ever go to DCC (which is less likely than a change of the scale I model) the gaps are there, so sorting out power districts won't be a problem for whoever buys my final layout from my estate.

One thing that bears mention, no matter whether you run DC or DCC, or how you wire the tracks on your layout.  Document everything!  Not only will that simplify troubleshooting, but it sure beats trying to remember just how the old track was powered when making changes to something that has been operating for years.

Chuck

[HEdward]

I found out a long time ago, when I was using common rail on a layout with a common rail connected throughout, that there was no way I could have two trains running in oppisite directions on the same track, even with separate power packs, because they short each other out.

 

I switched to dual rial block control and can now run two trains in opposite directions on the same track with two separate packs, using DPDT switches to control the blocks.

 

In common rail, one rail is intact all the way through. The other rail is gapped with power being supplied through a SPST switch. Try running two trains (DC) at the same time in oppisite directions on this track. It can't be done, unless the motor leads of one locomotive have been reversed. This is the reason why reversing loops require DPDT switches. When positive comes in contact with negative, you get a short.

Hi there.  Have you been taking circuit design lessons from Charles Eugene Doane?  Neither of you seems to know how this stuff works.  Chuckie thinks that you can use a diode to increase voltage and you've obviously gotten "common" wiring incorrect.  My last layout had a small yard, a passing siding and a long looping run.  While a train was on the long loop, another was going back and forth arranging itself and then waiting for the other to pass on the siding.  So you're telling me that the switching I was doing never happened?  I imagined it all?  I suppose next you'll send around an email like chuckie did asking for someone to shoot me. 

My next layout will be wired "common" for THREE train operation!  I'll be sure to have the fire department ready when I plug it in.

[TomDiehl]

I found out a long time ago, when I was using common rail on a layout with a common rail connected throughout, that there was no way I could have two trains running in oppisite directions on the same track, even with separate power packs, because they short each other out.

 

I switched to dual rial block control and can now run two trains in opposite directions on the same track with two separate packs, using DPDT switches to control the blocks.

 

In common rail, one rail is intact all the way through. The other rail is gapped with power being supplied through a SPST switch. Try running two trains (DC) at the same time in oppisite directions on this track. It can't be done, unless the motor leads of one locomotive have been reversed. This is the reason why reversing loops require DPDT switches. When positive comes in contact with negative, you get a short.

Hi there.  Have you been taking circuit design lessons from Charles Eugene Doane?  Neither of you seems to know how this stuff works.  Chuckie thinks that you can use a diode to increase voltage and you've obviously gotten "common" wiring incorrect.  My last layout had a small yard, a passing siding and a long looping run.  While a train was on the long loop, another was going back and forth arranging itself and then waiting for the other to pass on the siding.  So you're telling me that the switching I was doing never happened?  I imagined it all?  I suppose next you'll send around an email like chuckie did asking for someone to shoot me. 

My next layout will be wired "common" for THREE train operation!  I'll be sure to have the fire department ready when I plug it in.

As Jeffery hinted, I hope you're not planning any reverse loops or turning wyes. In them there is no such thing as a common rail, it would be called a short.

[Texas Zepher]

when I was using common rail on a layout with a common rail connected throughout, that there was no way I could have two trains running in oppisite directions on the same track, even with separate power packs, because they short each other out.

I switched to dual rial block control and can now run two trains in opposite directions on the same track with two separate packs, using DPDT switches to control the blocks.

What are you calling the "same track"?  There is no way this is possible with DC power regardless of how it is fed. The only way one can run two (or more) trains on the same track in opposite directions is to use some sort of command control system like DCC.

In common rail, one rail is intact all the way through. The other rail is gapped with power being supplied through a SPST switch . Try running two trains (DC) at the same time in oppisite directions on this track. It can't be done, unless the motor leads of one locomotive have been reversed. This is the reason why reversing loops require DPDT switches. When positive comes in contact with negative, you get a short.

Yes, I believe how the common rail method works is common knowledge, is  widely used, and the people responding to this post obviously understand it.

The rest of this just makes no sense.  Either the terminology for what is being described is imprecise, incomplete, or it is just wrong.   If what is meant by a “single track” is a loop of track with common wire blocks in it, then this is just wrong.  If it didn’t work as is claimed  and there is a “short” between the two power packs then it was done incorrectly.  And on a bigger level if the common wire system didn't work then  why would there be three decades of books and articals written about it and why does this thread even exist?  It obviously does work.  Stating that it doesn’t work, doesn’t change the fact that it does. On a personaly level, I’ve been running layouts with common wire blocks where the trains run in opposite directions on the same loop since I was in junior high (1972).  That is what passing sidings are for.  My last layout with block control had four power packs on a common rail and they all worked flawlessly running trains on the “same track” in opposite directions and without shorting each other out.

Any two electrical circuits with isolated power supplies can share a common wire without affecting one another.

In my opinion the real problem with a dual wire system, is when one wants more than two cabs.  To do three cabs one needs a DPTT (DP3T?) switch for each block.  For four cabs one needs DP4T switches, etc.  That gets real expensive really quick.  That is a reason I would content that common wire is more appropriate for large layouts.  How many small layouts need four to six throttles?   Of course the overarching issue to all this is that in this day and age of cheap readily available DCC equipment, why would anyone go through all these headaches with DC?

[Texas Zepher]

As Jeffery hinted, I hope you're not planning any reverse loops or turning wyes. In them there is no such thing as a common rail, it would be called a short.

You bring up the exception but that is only  in the "common rail".  But that doesn't mean a "common wire" system cannot be used to control it.  Once again, as many have been saying, people have been using common wire control systems for layouts with wyes, reversing loops, and turntables for decades.  This is not rocket science.

[HEdward]

I found out a long time ago, when I was using common rail on a layout with a common rail connected throughout, that there was no way I could have two trains running in oppisite directions on the same track, even with separate power packs, because they short each other out.

 

I switched to dual rial block control and can now run two trains in opposite directions on the same track with two separate packs, using DPDT switches to control the blocks.

 

In common rail, one rail is intact all the way through. The other rail is gapped with power being supplied through a SPST switch. Try running two trains (DC) at the same time in oppisite directions on this track. It can't be done, unless the motor leads of one locomotive have been reversed. This is the reason why reversing loops require DPDT switches. When positive comes in contact with negative, you get a short.

Hi there.  Have you been taking circuit design lessons from Charles Eugene Doane?  Neither of you seems to know how this stuff works.  Chuckie thinks that you can use a diode to increase voltage and you've obviously gotten "common" wiring incorrect.  My last layout had a small yard, a passing siding and a long looping run.  While a train was on the long loop, another was going back and forth arranging itself and then waiting for the other to pass on the siding.  So you're telling me that the switching I was doing never happened?  I imagined it all?  I suppose next you'll send around an email like chuckie did asking for someone to shoot me. 

My next layout will be wired "common" for THREE train operation!  I'll be sure to have the fire department ready when I plug it in.

As Jeffery hinted, I hope you're not planning any reverse loops or turning wyes. In them there is no such thing as a common rail, it would be called a short.

 

No wyes or reversing loops.  Those would be isolaed from the rest of the layout anyway.  My layout is alongside the bar and pool table and just beyond it is the sofa and video center.  When I find the disc, I'll post photos of the space empty.

[HEdward]

This is not rocket science.

Are you sure about that?  I've read a few threads where they talked about Mars and aliens and I even made a reference to a model rocketeer using DCC.

[Soo Line fan]

I found out a long time ago, when I was using common rail on a layout with a common rail connected throughout, that there was no way I could have two trains running in oppisite directions on the same track, even with separate power packs, because they short each other out.

 

I switched to dual rial block control and can now run two trains in opposite directions on the same track with two separate packs, using DPDT switches to control the blocks.

 

In common rail, one rail is intact all the way through. The other rail is gapped with power being supplied through a SPST switch. Try running two trains (DC) at the same time in oppisite directions on this track. It can't be done, unless the motor leads of one locomotive have been reversed. This is the reason why reversing loops require DPDT switches. When positive comes in contact with negative, you get a short.

I am somewhat confused here. Are you saying that with DC common rail wiring you cannot have 2 engines in adjacent blocks running in different directions unless you gap both rails? If that is what you are saying then you are mistaken. I have been using common rail for over 15 years on my layouts. You have to remember one thing; positive to negative only causes a short if the circuit is complete. Batteries do not short out when + to – occurs because the other side of the circuit is connected to a load, not each other. This is how every thing from a flashlight to an EV / Hybrid battery pack is wired. It’s called a series circuit.

 

On the subject of interrupting both sides of the circuit, it’s really is a waste of time. Home and Automotive wiring does not break both sides of the circuits. Neither should you. The more circuits and connections you add, the more potential failures can take place. This also makes trouble shooting harder, not easier. What about an open circuit? No one has mentioned this failure mode. You have introduced the potential for a lot of opens with the extra control circuit. Okay, what about shorts? You have also added the potential for a short at each one of the DPDTs and all the wiring routed there. If you have a short with common rail it will be coming from a particular block. Using your block switches you can easily determine the block that is causing the problem. As long as you have the ability to interrupt one side of the circuit, the short will be contained to the offending block.

Using a DMM will find it in "short" order. 

 

Jim