If you're going to do columns, seems like you could save on magnets by putting exactly one magnet, either in column A or B.
Though if you do that, you'd need to worry about ambiguity from trains going different directions along the track. With your scheme, you have to figure out which column is the clock, but that's doable because it's the one with all 1s. (If the data is all 1s, then it's not clear but which column is which. You can still figure out the train number but not which way it's going.)
With my solution, I suppose you could just add a leading pair of magnets in both columns, which adds two magnets (and the space for them) to the cost.
Also, I'm not a hardware guy, but it seems like magnet polarity matters for hall effect sensors. So maybe you could so something like a single column of magnets but flipped north up or south up depending whether you want to indicate a 0 or a 1. I'm not sure whether you'd need two hall effect sensors (in opposite orientations) or a different type of hall effect sensor.
Though if you do that, you'd need to worry about ambiguity from trains going different directions along the track. With your scheme, you have to figure out which column is the clock, but that's doable because it's the one with all 1s. (If the data is all 1s, then it's not clear but which column is which. You can still figure out the train number but not which way it's going.)
With my solution, I suppose you could just add a leading pair of magnets in both columns, which adds two magnets (and the space for them) to the cost.
Also, I'm not a hardware guy, but it seems like magnet polarity matters for hall effect sensors. So maybe you could so something like a single column of magnets but flipped north up or south up depending whether you want to indicate a 0 or a 1. I'm not sure whether you'd need two hall effect sensors (in opposite orientations) or a different type of hall effect sensor.