Which tags and tag groups need attention
Simply put, every single tag and hence also all tag "groups" (though we are not allowed to 'group' tags - see for example “Tagging” tags (so to speak)).
While I would very much appreciate any help with tagging, it is worth bearing in mind that many users do not, in practice (even if claiming to approve 'in theory').
We do presently have a Usage guide for every singe tag (almost) applied to any Open question - with something of an 'action plan' (see September 2016 Tag Rename & Synonym MegaQuestion) for a few remaining 'special problem' tags.
Regarding specifically public-transport I have chosen to classify the following as 'sub-categories' (only for my own purposes):
(so nothing specifically about modes of travel) and regarding trains:
Regarding modes of transport I have sub-categorised transportation as below:
There are various obvious tags missing from the above mostly because for them I chose to sub-categorise under the tag road-trips:
I will consider sub-categorsing visas (presently I have 180 tags lumped together under that category) once the renaming/synonymising is up-to date. There are a further 140 or so tags that I have not (yet?) categorised at all. Note that the tag information from SEDE is only updated weekly and even on update lags the 'main' database by days even so.
I would suggest the priority is to address questions tagged tagging on meta. If the Community will not indicate its preferences then there is no hope of significant progress.