How about variable catchment areas, depending on station type?
eg,
- a bus stop has a catchment of 2 tiles
- a bus station or a tram stop has a catchment of 3 tiles
- a railway station or heliport has a catchment of 4 tiles
- a small airport has a catchment of 5 tiles
- a large airport has a catchment of 6 tiles
The big question this raises is, what happens when you have interchange stations, eg when you add a bus stop to a railway station? As I see it, there are two options.
- The catchment for the railway station remains 4 tiles outside the railway station, and the catchment for the bus stop remains 2 tiles outside the bus stop - which in this case would mean no additional catchment, although it wouldn't always work like that.
- The largest catchment for the station group would apply around all constituent parts - so in this case, the catchment would extend 4 tiles away from any part of the railway station or bus stop.