I have been interested in making tight (read low resistance) connections between track sections for some time. I have tried using an ohmmeter with less than accurate or quick results. A few days ago, I rigged up a Kelvin connection for measuring resistance. It will measure as low as 20 micro-ohms. With this I have made the following measurements:
A good connection between track sections is 1.5 milli-ohms. This is with clean pins and clean openings in the two sections. Also, it is necessary to bend the ends of the section where the pins are installed to increase the contact pressure between the pins and the openings in the adjacent section. With the pins of a section pointing away from me, I bend the left rail where the pin is installed to the left and bend the center rail to the right. I also use pliers to form the ends of the open rail back to where they should be if they are bent out of shape.
Resistance of a Lionel O-31 curved section = 0.0035 ohms
Resistance of a good joint between sections = 0.0015 ohms
Resistance of a Lionel O-31 straight section = 0.0032 ohms
Resistance of a Lionel prewar 072 curved section = 0.0048 ohms
Resistance of a Lionel prewar 072 straight section = 0.0058
Resistance of a prewar American Flyer O-40 curved section = 0.0040
Resistance of a prewar American Flyer O-40 straight section = 0.0040
Resistance of a Lionel O-42 curved section = 0.0039 ohms
Resistance of a Lionel prewar 072 (711) switch = 0.0100
About half of the resistance of the 711 switch is the wire that connects from the rivet holding the fat rail to either of the other two center rails. This wire is apparently steel and not copper. Replacing this steel wire with a copper wire would reduce the resistance of the switch by about 0.003 ohms. With the steel wire, the resistance is 0.0047 ohms.
The four 711 switches that I tested had a significant amount of resistance between the rivet that holds the fat rail and the rivet itself. I soldered the rivet to the fat rail.
Making a Kelvin connection requires a constant current supply and a voltmeter than can read down to 10 micro volts. I am using a current of 100 milliamps and reading the voltage with a meter with a resolution of 10 micro volts. I multiply the voltage reading by 10 to get the resistance.
