I skimmed over the pages. It looks to me like there should be some kind of aggregation service (could be done by us) that queries the individual services and allows retrieving transport data for given geocoordinates. Or is there somethink like that already?

Kinda:

http://www.gtfs-data-exchange.com/
https://code.google.com/p/googletransitdatafeed/wiki/PublicFeeds

I think you will need to scrape feeds from the above URLs but also allow direct input of GTFS feeds.

It might be a good idea to talk to the spec folks about defining such an index, replacing PublicFeeds with gtfs-data-exchange.com and defining any extra interfaces needed, like search.

This is a basic feature every maps application should have. In most industrialized countries most trips are made in public transport systems. I'd say that way more people lookup routes by bus, metro or train than bicycle or pedestrian ones, at least in cities.
This request is 4 years old. Please, don' keeop ignoring or procrastinating this feature, it's an elementary one, Gmaps has it, Here Maps has it, and now that Marble is trying the mobile echosystem, it should be a /conditio sine qua non/.

eemantsal: I don't think it is appropriate to demand features like you have done. KDE folks volunteer their time, we users should thank them for their work and suggest possible uses of it but leave it up to them how they spend that time.

*** Bug 354335 has been marked as a duplicate of this bug. ***