TEM studies of a Bi-2223 tape have shown that amorphous layers from about 10 nm to more than 100 nm in width were present at c-axis twist boundaries in the highly textured Bi-2223 tape which showed a limited residual weak-link field dependence at low fields (μ₀H<0.1 T) followed by a slow exponential decay at higher fields. This kind of amorphous layer is likely to be insulating, and this strongly suggests that Jc is not limited by c-axis current transport as proposed by the 'brick wall' model. The most frequently observed low-angle grain boundaries are ab-axis grain boundaries containing no (001) planes. A modified 'rail switch' model was proposed with low-angle ab-axis grain boundaries at the switches rather than low-angle c-axis grain boundaries as suggested by Hensel et al.