Newman Atkinson says:

Dave, The wires underneath are considered you Buss Wires Large enough to keep from loosing power the farther away you get from your transformer or DCC system. You run a Buss Wire under each rail Both rails need to be connected to the buss at certain distances to make sure you are getting all the power to your tracks. I use 16 gauge trailer wire or better as the Buss wire and use color wire to make sure you connect to the same buss wire for each rail. If you are going very long distances some people will use a no, 14 or even a no, 12 gauge wire for their Buss wires. Your Drop wires or as you put them Jumper wires do not have to be heavy wire as you are only connecting a very short distance from the Buss wires under the layout to the track rails above the board (usually less than a foot of wire) Phone wire (usually a 22 or 24 gauge wire for those short distances works fine. Larger trains such as Lionel most likely need the heavier wire. If you only connect to one rail using a Buss wire , (as you were suggesting )the other rail will loose power the farther you are form your power supply. Every connection of the rail between where your engine is and the power supply you loose more and more power. A good example it your beginners simple loop of track usually has just one connection to the rails near the power supply. Your engine probably never gets over 10 feet from the transformer in either direction on that loop of track . Now take that same loop but expand it out to make a simple loop that runs all the way around your garage or basement you will find that with all the connections in the rails the farther you get from the power supply, the slower your engine will get. In DCC you may loose enough power that your engine DCC chip may loose enough power it will not function properly or at all on the other side of the garage. There would be enough power loss to keep it from working. Under standard DC system when the engine gets far enough away from the power supply you will notice the power drop by seeing it run slower.
So Yes you should run Buss wires for both rails and I use 4 to 8 feet for each connections to the rails. That is the rough distance if you were running on a portable modular system where the modules were 4 to 8 feet long. In the case of a permanent layout I use roughly the same distance so that I connect up to a flex track each direction from the connector and the same for the next section. and so on. If you have several switches make sure you have connections on each side of the switch of course allowing what you need for power control through the switch if directional power is involved. Now you did not mention what scale so the same thing applies to the AC power of Lionel (three rail) the farther you are away from the power supply the more power that is lost. I have been there and done that. The modern way is to run Buss wires and wire drops from your rail connectors to the buss wires. Do this with both rails. Use different color wire for each rail so you don’t get confused which wire you are connecting too and cause a short.
If you are running separate tracks such as double tracks use a color for each of the 4 rails you are following Color coding really helps when you are upside down under your table hooking up wires. If you have a switch yard and they all feed to the main track out( Same track) the wires for those 2 rails can feed each of the yard tracks.
My earlier days of model railroading I connected to one rail all the way around and for each block or positive side I fed to power switches and a separate wire to that block. It worked fine bu like I said you loose a lot of power in the distance. I also used to run phone wire for everything and that didn’t help. Buss wires is the way to go and it is actually easier to wire than the old way of wiring. Even if you are still running in Standard DC now, when you convert to DCC all you have to do is change your transforner for your new DCC power supply and you should be ready to go.
from Newman Atkinson

Newman Atkinson says:

For my wire drops I soldier the drops to the bottom of track connectors and install them so that each will contact the rail before and after the connector and do the same on down the line. Connecting the drps in that fashion will put the drops about every 6 feet which is good.
If powering a module section I ensure that the drops are connected to the rail just before and after the sectional track crossing to the next module. That way power will connect to the sectional section from either side of it.
By soldiering to the connectors instead of the rail, if changes need to be made then breaking soldiered connections will be minimal. Not soldiering the connectors to the rail does the same.
Many will suggest to soldiering to the bottom of the rail which is a good thing but I find unnecessary in most cases. If you are connecting flex track together to install in a curve the soldered rail connector in a continued curved rail rather than an uneven joint. Connecting a wire drop on that connector might as well be since soldiering is necessary anyway. It has been sof so good for me. from Newman

Rich The Engineer says:

Short answer, yes. Both + and – feeder wires, aka “buss” wires, should be used for best results.