By my current udnerstanding, Goods can only enter or leave the train-network at Railheads or Stations. ...
Troops can not take stuff directly from the rails. Assets might be able to, I have not tested it for a while.
Nope, you have it wrong. Items can enter and exit, and supply can be drawn using any point on the rail that has sufficient logistics points. Railheads & rail stations are only necessary at the ends of the rail, to eliminate the 90% penalty for rail logistics that's heading towards a direction that does not have a railhead or station at the other end.
Since I've seen you be very insistent on this when corrected on it by other people, and I'd prefer misinformation about the game mechanics to not circulate, I went to the trouble of making test case for the mechanics. You can check out this savegame if you don't believe me.
In the test save, there is only one truck station in the regime and it is mothballed. There is one rail station, generating 2000 rail points, and one railhead, but they're both 6 hexes out from the nearest town. Nonetheless, the 475 surplus food produced by the level 2 farm dome in the town of Angelfire is being delivered to the SHQ without difficulty, I can send colonists to Angelfire from the SHQ at the full logistics bandwidth and not zero or 10%, and if I order the Independent Artillery Division at coordinates 119,6 to disband, it sends all its troops to the SHQ after ending turn even though their combined weight exceeds 10% of the available rail logistics, then returns them all the the turn after if I order it to cancel disbanding. The Logistics Used Pts mode shows it drawing the logistics points needed to move the subunits directly from the rail, not the railhead two more hexes away (if it had to draw its supply directly from the railhead, it would not be in green logistics range and would thus be unable to receive replacements).