Well, that doesn't actually mean my idea is a bad one, it just means that,
since all tube lines (okay, not quite all) would count as high-frequency
and none of the NR lines would (okay, not quite none), it's actually
exactly equivalent (okay, not quite exactly) to drawing tube and NR lines
differently! Still, it's the principle of the thing that matters.

The similarity in off-peak daytime frequency is not enough. A
significantly different feature of Underground lines is that they run
all day every day at high frequencies in both directions.

NR routes which have high frequencies don't necessarily run all the
time and nearly always close down hours earlier in one direction than
the other.

The types of service are so different that they can't be shown in the
same way, certainly not on the grounds of daytime frequency.