Allen,

 And to add to this excellent summary ... there was a fad of painting up some 
regular box cars for "expedited" service.  The SP Overnight, B&O Sentinel,
GN Orange and Green, etc., etc., etc.
  All of the RRs actually started out using those particular cars in some
special service (usually on their own rails) ... and in about 2 years time
those cars became "any box car - any load" cars and the paint on them
was merely an advertisement.  And, frequently, they didn't actually
provide better service than any other even in their earliest years!
  Think "marketing that doesn't actually mean anything" and you have it.
A few RRs actually put considerable extra effort to provide the advertised
service ... but the realities were that it was too costly and there were so
many reasons why it wasn't really happening that they quickly 
abandoned the attempts. 
  It was quite common for one of these cars to find its way into a
regular freight - and when that happened the "special service" they
were meant to be providing was out the window.  Likewise, it was
likely that they would not be loaded with the 'right stuff' to justify
special handling and again the use of these "special" cars was
controverted.  Finally, as soon as one of those cars left the home
road's rails there was zero guarantee they'd get any kind of 
special handling.
  I am not talking about "express cars" - I'm talking about the 
regular box cars with special paint.  So if you are modeling the
correct year(s) and you want to use these cars - go for it - or
you can just use them in your regular service as "bright
shiny objects".  If you are into "era correctness" then you need
to consider the year that those cars were painted in that scheme
(even/especially if they aren't for "your" railroad).
                                                                                      - Jim