Why 16.7 Hz is different
At 16.7 Hz, induced-current shielding is far less effective than at 50 Hz. Ferromagnetic materials and geometry carry most of the mitigation burden.
Return current paths
Fields depend strongly on how return current divides between rails, return conductors and earth. Modelling this split is the single largest source of prediction error.
Overbuilt developments
Buildings above tracks need shielding integrated into the slab during structural design — retrofitting is often physically impossible.