I think you’re right Azrael, the AI cannot build roads over rail lines either. They will only take advantage of existing roads. When I lay out a station, I deliberately add some transverse roads before creating the rail lines so that development at least has a chance of reaching around to the other side of the tracks.
But in most cases I found that the development was pretty minor and of low economic value. To improve it I had to build roads closer to the station ends, ones that are closer to the heart of the city. From reading the developers’ log, I think the main factor influencing the amount of development is travel time. So closer roads, or quicker in-city transit, will help.