hello, Jeroen. I noticed your edits on a couple of lines. After posting here, I changed the Shrewsbury-Chester line, which immediately provoked a discussion on whether this was a main or a branch line https://www.openstreetmap.org/changeset/85440239#map=10/52.8533/-2.9105. There doesn't seem to be much consistency in these classifications. This route now appears on more zoom levels on OpenRailwayMap.
This line does not have any relations apart from a route=train one for Chester-Wrexham as part of WCML to Euston. The neighbouring Crewe-Shrewsbury though has 2 route=tracks. This seems to me to be a good way to group ways together, but doesn't appear to be consistently used either. route=train is nice to have, but I would imagine is a large amount of work to maintain, as services, unlike tracks, change frequently.
I've been looking at Overpass Turbo to familiarise myself with the API. Using 'out tags' looks like a good way of limiting the output to just a list of which tracks don't have a usage tag. I'm reluctant to install Java just to use JOSM, but may well take a look. Thanks for your suggestions.