i think you're concerned about connecting the + side of one cab (power supply) to the - side of the other cab. This isn't a problem. This isn't a potential problem until you connect the other sides of the cabs together.
short answer is don't and there is certainly no need to.
the diagram below illustrates a layout with 3 blocks: the oval and 2 spurs. There's a gaps in the non-common rail in each spur isolating it from the non-common rail of the oval. One (-) side of each cab is shown connected to the common rail. switches allow either (only one) cab to be connected to the common rail.
when a loco is on the rail within a block, it is (sigh) connected to both cabs, but only to both terminals of one cab. Current can only flow from the cab connected to both rails. Current cannot flow from the cab with only one connection.
yellow_cadAt each wire drop from the the track, I intended to connect wires from both sides of the track. Would it be correct to then connect my A bus line negative wire to my B bus line negative wire, and attach the combined negative wires to the common terminal on the reversing selector?
the diagram below unnecessarily shows the - side of each cab connected to the common rail (black). The polarity of the cab terminals needs to be reversed to make a locomotive change direction. It makes not difference if the + terminal of one cab and the - terminal are connected together. current cannot flow unless the other terminals of both cabs are connected together.
so if cab A is connected to a spur and cab B to the oval, the voltages on the non-common rail on either side of the gap may be different. So far, no problem. But if a wheel of a locomotive bridges the gap (i.e. touches both rails at the same time), there is a potential problem.
so there's the potential for this problem if a loco staddles the gap and connects both rails with a wheel or thru the frame of the locomotive.
yellow_cad If that is OK so far, how do I slide switches that control the frog polarity? Can the + and - frog drop wire (from the slide switch) be connected to the A or B bus line depending which block they are in. I am just not sure about the common wire in all this.
not sure i fully understand what you're saying. The slide switch should be connected to the adjoining track connections and the frog connected connected to one track or the other. There should only be one cab connected to both sides of the track (unless a loco bridges a gap).
greg - Philadelphia & Reading / Reading
You are building a complicated layout. LION'S layout looks very complicated, but when you lay it out, it is a simple single track nine miles long.
So the left rail is the COMMON Ground, and the right rail with +v dc moves the train forward, -v dc moves the train in reverse.
Run your finger along the left rail all through your layout. If it ever ends up on the right rail then you have an issue that you must deal with.
Take if you wish two or three power supplies. Put the (-) pole to ground. Run the other pole to your right rail. Break it up into whatever blocks you require. The reversing switch will operate as expected, and the train will move in accordance to your power suply.
IF YOU USE A COMMON GROUND system, besure that you DO connect it to a HARD (building) ground, otherwise you will collect stray voltages on that wire.
LION also uses that same common ground for all of his circuits, lighting, sighnals, realys, detectors etc etc. Him has lots of transformers for many purposes.
ON REVERSING SECTIONS, the loop must use a dpdt switch since the left rail becomes the right rail depending on the position of the turnout.
LION gets around this by applying power to the mane lion and allowing the position of the turnout apply power to the (fully gapped) reverse section.
LION has a double crossover, and rather than using realys et al to enervate it correctly, him just isolates it without power. 24 wheel power pick-up negates the issue, but a single locomotive will of course stall. LION does not run any of those.
ROAR
The Route of the Broadway Lion The Largest Subway Layout in North Dakota.
Here there be cats. LIONS with CAMERAS
I am wiring for DC, but want to keep the option of later converting it to DCC in mind. My layout for discussion purposes is small (appx. 5 ft x 3 ft), and consists of a figure 8 inside of an oval connected by a double cross-over (Shinohara that is insulated all the way across the middle) at one point and a reversing loop at another. All RR switches (Shinohara) have live frogs with slide switches to control polarity and movement of the switch. My intention is to make the figure 8 a block and the oval another block with the insulated double cross-over separating the blocks. For track controls, I plan to use an Atlas reversing section controller and an Atlas block selector. I will connect my A bus line to the A block track in two places and the B bus line to the B block track in two places, and the reversing section track wires will go straight to the reversing controller. At each wire drop from the the track, I intended to connect wires from both sides of the track. Would it be correct to then connect my A bus line negative wire to my B bus line negative wire, and attach the combined negative wires to the common terminal on the reversing selector? If that is OK so far, how do I slide switches that control the frog polarity? Can the + and - frog drop wire (from the slide switch) be connected to the A or B bus line depending which block they are in. I am just not sure about the common wire in all this. Any help would be greatly appreciated. Jim