I tend to wire my speakers in series because it presents a 16 ohm load to the amplifier, thus decreasing how many watts the amp puts out. This does two things: 1.) it prevents the amp from damaging the speakers when the max wattage of the speaker isn't known (most of the ones I've used are 2 watt max, but many are 1 watt max), and 2.) it keeps the sound from being obnoxiously loud. The sounds coming out of our locomotives should be pleasant to the ear, not migraine-inducing.
-Kevin