Kline engine parallel/series switches?

"Series" places the two motors in series wiring. + on the reverse board output to + on one motor, - on that motor to + on the other motor, and - on the other motor back to the reverse board.

"Parallel" places the two motors so each + gets power direct from the same source on the reverse board without sharing voltage with its partner.

What that means to you is that the engine's motors will run faster in "parallel" (the default wiring scheme for all K-Line locos with similar drivetrains). Maybe too fast for your tastes if you use a postwar transformer that won't output any less than 5-6 volts. Switching it to "series" will slow down the motors, (increase the voltage requirements of the loco for a given speed) at the cost of some pulling power (i.e. a little easier to stall the motors under a heavy load)

The On/Off switch determines whether the reverse unit operates as normal (on) or locks the locomotive in the direction it was moving last (off)

---PCJ

I think there is an "oops" there. 

Parallel has speed, series has torque and with it, better creeping ability. Series needs more voltage at X speed.

A small transformer works best with parallel since it uses less voltage overall and small transformers often have less max voltage. If you have enough power on tap, series is worth a go, especially if you run prototypically, vs 200 scale mph.

I'm a high speed looper

Series has an additional advantage in that the loco will be less susceptible to speed variations on sections of track with lower voltage, poor connections, or further from the power connection.