[cferry]

However in Cologne, to my big surprise, trams, despite running in the streets and in a real maze of crossings underground, manage to always be on time when I use them. S-Bahns on the other hand, seldom are...

Stadtbahns use the "U" sign for U-Bahn when only part of their network is underground; I guess this is to avoid confusing riders.

[Tainnor]

You might have been lucky. The "metro" in Cologne is notoriously unreliable, to the point that there's even a word "KVB minutes" to explain why sometimes the train will be arriving in 2 minutes for about 10 minutes.