[openrailwaymap] Adding usage tag for multiple ways

I've been looking at OpenRailwayMap for GB, where I live. I thought at first that it was missing numerous lines, but on investigating further I find that this is because large numbers of ways have no usage tag, and it looks like OpenRailwayMap only shows lines which are not tagged as 'main' or 'branch' when you are zoomed right in. Having to edit each way separately would be a large amount of work, especially as lines often consist of multiple ways. What's the simplest way of (a) finding which ways don't currently have it, and (b) adding the appropriate tag? Without it, I fear that OpenRailwayMap is of limited use.