Are you talking about Atlas switch machines or Snap Relays? Atlas switch machines have a throwbar at one end and three terminals at the other. The center terminal is the common return and the other two connect to each coil that throws the throwbar over. Atlas Snap Relays have NO throwbar. They have the same three connection screw terminals on one end, but two sets of terminals on each long side. Each of these terminals have three contacts. This essentially makes the Snap Relays into two SPDT switches, or one DPDT, depending on how you wire them to whatever...polarity reversal, signals, etc. For example, if you are trying to control the polarity of the frog on an Atlas Mark IV (or any other turnout frog that is insulated from the adjacent rails, you wire the relay coils in parallel with the switch machine coils. Or you could use the Relay contacts to control signal lights or reverse the polarity in a reverse loop. Just as with the switch machines, the coils on the Snap Relays must have MOMENTARY power applied ONLY, or you will burn them out. This can be done with Atlas control contacts, or a simple pushbutton wired between the power supply and the relay.