Since going uphill will use full power, downhill is the only direction that would require the voltage reduction diodes. But you still need 2 sensors. One at the top of the hill and one at the bottom. These could be anything that delivers a positive DC voltage to a 5 or 12 volt relay module. Noise should not make any difference because the relays are setup to hold their position from one sensor to the other.
When the train reaches the top of hill sensor, the first relay kicks in. That sends power from the NO output to the NC input of the second relay. Since the second relay will be off, the NC and COM points will be connected. The COM connection runs to the COIL of the first relay so as to maintain power after the train gets past the top of hill sensor.
The "downhill" signal controls the third relay to route either full strength transformer power or diode reduced voltage to the center rail of the downhill track section.
When the train reaches the bottom of hill sensor, the connection between NC and COM on relay 2 will be broken and the COIL on relay 1 will disengage. This will restore full track power to the downhill section of track. If the train turns around or even backs up, it can engage the hill with full power.
Please examine this to see if I missed anything. If I did, please let me know. I'm sorry that I'm not in a position to demonstrate or test this out but in theory it should do the job.
