Chagrin:
Assuming you've rigged up your IR reflectance sensor right (and that's a big assumption), it should trigger low when the white label of the barcode starts as hopefully the barcodes are the only reflective thing passing over the sensor.
I've not rigged anything yet, just trying to sort out the feasability 
Hopefully, the barcodes will be the most reflective thing passing over it. I could paint the remainder of the underside of the wagons matt black.
Thanks for your reply. I'll certainly consider it.