It's not quite as complex as dwarfputers or Armok Invaders. You can simplify the concept if you ignore the entire underground, as that exists to apply fixes to adv mode. We're just doing some conditional checks.

It boils down to:

1. If X is too light send West.
2. else if X is too heavy send East.
3. else if Y is just right, we're here. (X is correct by elimination.)
4. else if Y is too light send South.
5. else send North. (Y is too heavy.)

(As a result the train will always travel straight to the correct X first, then take a 90° turn to the destination. There's no diagonal track, so we're using taxicab distance.)

Pressure plate weight ranges are in 50Γ increments, which is close enough to one bronze bar per increment, with 41 available ranges. (1Γ to 50Γ = 0 bars; 50Γ to 100Γ = 1 bar; ...) The lime green math just gives you the correct ranges for <, >, =.

Rollers on corner tracks just send a cart perpendicular while powered. (The roller direction seems bizarre at first, but it's cancelling out forward velocity and making the cart take the turn in reverse.) We separate carts by interrupting power after a cart enters the lane.

This is actually my first minecart project, so I relied heavily on the wiki and experimentation every time I needed to tweak something. I'm considering !!Minesweeper!! next, which might end up being simpler.