It seems like some combinations of vehicles will result in some very strange numbers being calculated.
I ran it, and it seems to behave all the same with 8km/h.
(http://i199.photobucket.com/albums/aa131/AEObikes/simutrans/th_simscr57.png) (http://i199.photobucket.com/albums/aa131/AEObikes/simutrans/simscr57.png)(http://i199.photobucket.com/albums/aa131/AEObikes/simutrans/th_simscr58.png) (http://i199.photobucket.com/albums/aa131/AEObikes/simutrans/simscr58.png)(http://i199.photobucket.com/albums/aa131/AEObikes/simutrans/th_simscr59.png) (http://i199.photobucket.com/albums/aa131/AEObikes/simutrans/simscr59.png)
Thank you for the report. This appears to affect the physics code; I have asked Bernd to look into this.
I'm going to look into it ASAP...
Thank you very much :-)