Milk Train Phrase

I've heard 'milk run' in reference to a delivery route that's less than challenging.  Boring!  Milk train, on the other hand, has something to do with actual milk.  In 1853 any train with perishable cargo got the right of way on the rail lines. Fair enough. Around 1910 that condescension comes in to play.  At that point a milk train was a poky old thing that stopped at every single station to drop off and pick up milk cans.  You can see how this might be construed as dull. 

Source:  I Hear America Talking p.209