Why not just add a resistor in the power cable to the motor? You should be able to get as many engines as you like to match in speed. Resistors are available in many different values -- just try different values until you get the speeds to match or buy a potentiometer (variable resistor) and adjust for value needed . Cost is very cheap for either. (a few cents for a resistor or a buck for a potentiometer). Just add one to each of the faster of the engines. (Values more than likely will be different for each one). Nothing needs to be done to the slowest engine. When testing for same speeds make sure that you check on the same section of track using the same controller at the same setting. (Different controllers can put out different voltages at the same settings, which will vary speed.) If you are trying to test on parallel tracks, you may encounter problems due to different voltages due to different distances from the power supply and therefore different speeds. (In other words, when testing make sure everything else is the same.) Hope this helps.