Not sure of the explanation, but some bus blinds showing stations as
destinations *do* show the rail (and sometimes Tube) symbols. For
example, the 18 to Euston.

Interestingly, though unrelated to the original question, some (but
strangely not all) 258s to Watford Junction show the Tube symbol as
well as the NR one - is this in anticipation of the going-ahead (and
timely completion) of the Croxley Link?

Illustrated he
http://www.busesatwork.co.uk/images2...58_VTL1219.jpg