A mod to do this would require modding the game engine’s source code itself which isn’t possible at the moment. Hopefully, the devs might consider allowing us to mod that in the future (or greatly expand modding capabilities) like a number of other games have allowed (like Civilization 5, Mount and Blade, etc). At the moment, you can only mod the physical objects in the game, like buildings, vehicles, trees, people etc and their costs, stats, speeds, etc.

You could go through every vehicle you don’t want and turn off their introduction year, or set it to something ridiculous like the year 5000. Same for buildings and roads, but that would be a very large amount of work since there are a lot of objects in this game.

You can reach this state “de facto” by editing the resource files and ensure that anything created before year X is permanent ( just set yearTo = x property to 0 ) and anything introduced past that year is never introduced ( set yearFrom = x to absurdly high value – say 99999 ). Time will pass normally, but your technology will be frozen at a given year.

That’s exactly what I want, I don’t’ mind if the years keep incrementing.  Much thanks.

Where do I find these resource files, and can I make this into a mod for easy enabling/disabling.

In that folder are more folders of vehicle types. Each folder has many .mdl files that can be opened with a text editor (recommend notepad++).  Make sure you also do the civilian vehicles or you’ll go bankrupt.

You can also edit buildings and roads which are all located back in the <install dir>\res\models\model folder. Then you can stop the new roads, bridges and buildings from appearing.

..or you can use the timeless mod

Timeless mod is actually the complete opposite of what I want.

If i wanted to stay in 1860, i would get timeless mod and buy only the vehicles from that era. I don’t understand what are you trying to acomplish here?