I have a Robert Chipman book on transmission lines that mentioned this issue. As I recall, the loading coils did not improve high frequency transmission, but rather, just the opposite. The loading coils were removed for that reason.
I'll check my reference when I get home tonight, but I seem to remember that the reason for adding the loading coils was to lower distortion. Sir Oliver Heaviside determined in the 19th century that distortionles transmission takes place when R/L = G/C, where R, L, G, & C are the transmission line series resistance, inductance, shunt conductance, & capacitance per unit length resp.
The loading coils make it easy to optimize the ratio to get low distortion. They were added around 1925. But they acted as a low pass filter, attenuating hf response. They were removed around 1940. From memory, that is what I recall. At home I'll check the reference material and post a correction if needed. BR.