It should run half as fast for the same track voltage as it did with the motors in parallel. Am I missing something about the question? For the same track voltage, speed should be half and current draw should be half. These figures probably fall apart at extremely low track voltages, around 3 volts or less.
It will also be less susceptible to low voltage spots in the track by a factor of four over the stock, parallel wired locos.
When wiring in series MORE voltage is required, because each motor only gets HALF of the track voltage.
With every discussion of wiring the motors in series, someone always chimes in with "if one truck slips, the other will stop." While that is true in theory, I have never seen it happen with dozens of series-wired locos and over at least fifteen years of service. So don't worry when someone says that.
