Regulating the speed of an electric locomotive is very difficult. Tyristors and transistors revolutioned the area and the problems are gone. But in the early days there were no tyristors, let alone transistors. The the slip ring AC motors were one solution.
If I understood your description correctly (big if!), all three rods will go into the same saline bath. There is no return conductor, as the currents from the three phases cancel out in the middle. This is electrically the same thing as putting a resistor in each of the windings. Logically, is doesn't matter where in the winding you put the resistor.
If you had put the saline resistors on the other side of the windings, you would have needed a separate bath for each phase.
