I do not see how that would work since the trains run in both directions, you would have to manually worked the throttles. Also pickup rollers will jump the center blocks paralleling the transformer taps.
Easier to use one tap with a voltage dropper. Relay logic could establish train direction using 2 relays. The method is shown here for directional crossing gates.
www.jcstudiosinc.com/BiDirectionalCrossingGate
Dropping voltage with diodes and bridge rectifiers is shown here
www.jcstudiosinc.com/BlogShowThread?id=413
I think you would need 4 relays in all and maybe 5 bridge rectifiers to drop voltage.
From that cascade of diodes you would have 3 voltages, no drop,for uphill,-3 volts for flat surfaces and -6 volts for downhill. These voltages would be adjustable to suit needs.
One 18V transformer tap could run the relay coils. Another tap would supply track voltage in series with the diode cascade. If you could make an outside insulated rail all the way up the spiral or grade,I think you could do it with 4 relays. I do not know how easy this would be to do with fast track. Other wise you could do it with 10 relays, a much more cumbersome latching system with short isolated out side rails at the grade approaches. The 2 direction relays would determine whether the 2 grade occupancy relay contacts cut or increase voltage on the grade when occupied by the train.
With this method,if only 1 train runs on the loop, no center rails would need to be isolated,thus no roller jumping problems,only the outside rail on the grades and 2 short sections at a distant place which would work the direction relays.. The power to the track would be determined by the position and direction of the train.
Dale H