I had a (very) profitable line set up between two towns that was running full trains (60 passengers on every run). When I extended it to include a third town and added the extra station in it completely crashed. Even on the section of the route that wasn’t affected by the addition (I added extra trains to compensate) it immediately went to zero passengers and had to build up again from scratch.
I’ve seen this behaviour when adding pretty much any new network feature, be it a truck stop (cargo on the road disappears) or a station to a line. It’s not only infuriating, it’s hugely costly – my line was long distance and costly to run and I went from hugely in the black to making over a million a year loss on that line.
Adding a stop to an existing line, or placing a truck/passenger stop on a road shouldn’t be so destructive to existing traffic using those features.