Hmm they look old, should be the right ones.
But I have a hot trap:
The oldest car has starting year 0 which maybe is the reason it doesn't work. (Found in C:\Program Files\Steam\SteamApps\common\Train Fever\res\models\model\vehicle\car\ford_fordor_1934.mdl).
I will change it to 1850 and give feedback.
Edit: 1850 was nonsense,then cars would eb avaliable from the start. So I tested it with 1910. At month change the game crashes, I think that the reason is that it doesn't find a car to put the people in. I also tested it with a new game: At 1.1.1901 the game crashes, maybe that's the date where cars are avaliable.
It looks like it is like you said: car using is coded somewhere else. My workaround now is to limit the old car's speed to 18 km/h. Maybe then they won't use it this often.
But for testing purposes I will limit the speed to 1 km/h^^
Edit2: I limited the car's speed to 3 km/h (topspeed). After two months ingame the cars recognized the change. Most people abandonned their car immediately. Two lonely souls traveled to their destination for five years and holded up my ultra fast horse carriages
Since now walking is faster than driving this car, I don't see cars anywhere.
I think this is a nice workaround.
By the way: Gwindas mod makes the same changes like your code, BR146, at least for models. So I will continue to use Gwindas timeless mod (because I started my save game with it).