U Flight Lands In Grand Juction Co

[jimntx]

I think it originated with the railroads (in the manner you mention-- if there was a reason to stop, someone would signal or flag from the platform; if not they just kept on going), and the airlines adopted it.

You are correct. (I am descended from a number of RR men.) Smaller towns along a rail route would not have scheduled stops, but the train would stop if the station manager put out a flag indicating there were passengers or freight to collect.