There was a full train reserved only for holders of air tickets. This called Lufthansa Airport Express was a kind of "wet leasing" a train. At that time the high-speed rail Frankfurt-Cologne was not finished and this train passed the romantic Rhine valley. Therefore an attraction for tourists!
A quick link:
http://de.wikipedia.org/wiki/DB-Baureihe_403_%281973%29
The used a for that time hyper-modern train type which German Rail had only 3 of. Finally they got LH livery and interior. Due to logistic reasons and the Cologne-Bonn Airport being kept open they distincted between QKL/CGN.
During trade fair periods there was also an additional stop at Cologne-Deutz just 3 minustes from Cologne Hbf(QKL).