I think this is pretty useful 
Indeed, that's a huge improvement when it comes to optimising networks to improve the use of public service.
The last thing I am missing for that kind of work now is a new stop layer that shows some kind of the potential of stops.
However, it's not too easy to define what that potential actually means.
It should be a measure for how many pasengers a stop might attract compared to other stops assuming the same quality of service.
that means, it's some kind of passenger generation and demand within catchment area but obviously, simply summing these up is quite meaningless.
E.g. When a stop is placed in the middle of nowhere with densely populated areas around the border of the catchment area the stop would calculate a high potential, although that stop is actually useless.
We could just sum up passenger generation and demand of buildings where that stop is closest to, but that is not quite meaningful either.
Imagine a stop placed quite well in a densely populated area being surrounded by other stops.
Generally, a denser placement of stops will report less potential for each stop.
Imho a good definition of stops potential would be summing up generation and demand of all buildings in the catchment area but weightening them antiproportionally to their distance. The closer the building is to the stop the higher its weight.
Something like (generation+demand)/distance might work out well.
Something like (generation+demand)/(a*distance+b) with a constant a and b could be used to adjust the weightening curve if it turns out to attract too many
where a and ba are constant adjustement factors to control how much
Maybe it even makes sense for both layers to co-exist:
If you want to link your network, you usually care about two things.
1. You don't want to drive large detours
2. You want to stop where you can attract as many passengers as possible directly, so these passengers don't need to transfer.
The antiproportinally scaled sum will give you a god guess of the latter.
On the other hand, a quite close stop distance might be worth if the area is densely populated, but not quite worth if the area is barely populated.
You might consider increasing or decreasing stop distances on a line.
The summed up passenger generation and demand will give you a good guess of how many passengers will actually use your stops, assuming surrounding stops have the same level of service.
If that number is too low, you might want to increase the stop distance to save maintainance. If the number is quite high, you might consider adding more stops.
Would it be better to integrate the "Passengers" and "Mail" buttons and switch between them using the goods category filter?
Imho, yes as it's more consistent.
On the other hand, that only makes sense if we also got cargo catchments, otherwise the filter itself is kind of inconsistent again.
n that case the button label should be something like "Station accessibility"
I would rather stay with station coverage, as station accessibility is rather about accessibility of vision-impaired people, wheelchair users and so on, at least that's my association as a non-native speaker.
Station coverage in combination with the new colors reads like "that building is perfectly covered by stops" to "that building is badly covered by stop", which is fine imho.