Is as easy as having entry points on different levels (I understand there to be plans to allow for underground slopes in the future, but this may or may not happen). It is also as difficult as trying to work out what square the cursor is on when it is at ground level two or three levels above the track. This second point is being dealt with by the new underground mode, but I am not sure as to the status of that.
My network schemes
I start with building lines for the initial industries. If one of the rail routes connects two towns, I run a passenger train (I generally don't run passenger busses as I have a 2048x1536 map with 48 towns) once I have a good initial income, I then start to complete my rail lines, aiming for around 1/6th of the towns being 'hubs' where multiple routes meet, and having express train between them. On a line between two of the hubs, I have two local lines, each covering one half of the route, and stopping at all stations.
At least, that is the theory. In practise I have semi-fast trains, which connect some stations along a portion of a route, branch lines which connect to small towns not on a direct route between two hubs, and 'sort-f hubs' which are the end of one route, but in the middle of another. I also end up with goods-only bypass lines, passenger-only bypass lines 6-track main lines which cannot cope with the traffic level, and a 7 platform station at a relatively small town (at one end of the platform there is a junction where a six-track main line and a 4-track main line join). Within towns there are multiple stations on the main line through (with platforms off the main line). These are served by the local trains. Froms these stations I have mostly trams (I started my current train with pak128-gb before the omnibusses became available), plus some busses and even some monorails connecting outlying sections of the town, but if the bus or trams routes become too long, I build inner-city railways in the longest directions to reduce the length of the tram and bus routes.
Severous: I am impressed. I only have 1200 vehicles (but I do have 600 lines, and many of my trains have 15 carriages or 22 wagons)
I'd post some pictures, but I have no current access to somewhere to store them.