What happens if the train is going in the other direction? Then this uphill / downhill terminology is bogus with respect to the track. You need to be able to turn it around. Switches would work if you don't mind the responsibility of manual operation. But some track sensors and relays could change the orientation of the diodes based on the trains direction.
I would think that a top-of-the-hill / bottom-of-the-hill sensor and a cheap asian relay board to steer the diodes would be all that is needed. Nothing fancy. This should be standard apparel for any conventional layout that has grades to manuever.
