I have a world with a large conurbation (you can check it here: http://forum.simutrans.com/index.php?topic=14855.0
) and I've divided my transport network into four systems:
4. railways - connecting towns and the metropolis
3. regional buses - connecting villages with towns
2. city buses - connecting suburbs with downtown
1. trams/ downtown buses - operate in the city center
In the future, there might be the fifth level, airplanes.
I don't really care about the coverage, so I have the minimum distance between two tram stops 20 blocks, between two bus stops 25 blocks and between two railway stations 65 blocks.
also, I've got a question for you, how do you name your lines?
I use numbers and then the first stop and the last one. For trams, I have numbers 1 - 59 (but now there are just lines 1, 2 and 3), for downtown buses 100 - 199, suburb buses 200 - 299, regional buses 300 - 499 and railways start with 'S' and then with a number from 1 to 99.
For example, I have the line 103 Jilmovka - Bělehradská, so that's a bus starting downtown at Jilmovka, ending uptown at Bělehradská. Or there is the line 432 Bolice, Otvovická - Kramolín, Zdechovická. So you know it starts in the town Bolice and goes to a village named Kramolín. And for trains, S1 - S9 are the main railways and S10 - S99 are the regional ones. So when you are at the Ryjice train station and you see that there are lines 'S2, S20, 421, 422, 423', it stands for three regional buses, one main railway and one local railway.