Tony,

This has been a favorite topic as I have been adding placards and car routing cards where appropriate as part of my final detailing process.  Even on cars that have neither, I like at add residue:  scraps of former placards or route cards.  I chop up bits of paper with a razor blade until I get a fine confetti.  Add this to a little dab or Elmer's glue and mission accomplished.

I'm sure you are aware that some roads (NKP, C&O, MWR to name 3) mounted their door tack boards off center toward the opening so a man standing in the car could reach the tack board.  I like to install placards on these car doors as if a man standing in the car and reaching around did the work.  That makes the placards crooked and oriented toward the door opening even further.  

Jaeger used to make a nice selection of placards, but last time I looked they had been dropped from their line.

Mont Switzer