in a first step you could take a planet file and filter out only the objects you need. Then you could import the filtered data into a database, e.g. with osm2pgsql. With a PostGIS database you are able to do many different geographic queries such as getting all nodes that are part of a way or calculating way lengths.
Thanks for that. In that case I'll stick with the "path" tool in Google Earth though.
I had just hoped that someone would have actually thought of implementing a user interface for extracting information from Openrailwaymap. But, well, "web application" and "user interface", those two things just don't seem to fit.
Sincerely,
Wolfgang