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):
atac
london-underground
oyster-card
ratp
tallinja-cards
tfl
(so nothing specifically about modes of travel) and regarding trains:
amtrak
china-railway
deutsche-bahn
eurail
eurostar
high-speed-rail
indian-railways
interrail
japan-rail
jr-passes
national-rail
nederlandse-spoorwegen
shinkansen
sncf
tatkal
tgvair
thalys
the-canadian
train-stations
trans-siberian
trenitalia
trenord
via-rail
Regarding modes of transport I have sub-categorised transportation as below:
ferries
airbus
aircraft
air-travel
boeing
freighter-travel
gondolas-and-cable-cars
helicopters
hitchhiking
intermodal
night-transport
riverboats
sea-travel
ships
submarine
There are various obvious tags missing from the above mostly because for them I chose to sub-categorise under the tag road-trips:
automobiles
avis
bikes
buses
car-pooling
car-rentals
child-safety-seat
driving
driving-licenses
greyhound
i-80
marshrutkas
motorcycles
offroad
ride-sharing
road-signs
rvs
scooters
taxis
uber
vignettes
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.