Transport Tycoon Forums

I would like to know into which folder i should put my locomotion (steam edition) mods into. If needed, I also have OpenLoco already.

Assuming you're running Windows, the Steam version installation can be found at C:\Program Files (x86)\Steam\steamapps\common\Locomotion. If this is your first time using a mod, you should first create a backup of your Objdata folder and move the copy somewhere else so they don't get mixed up.

If the mods are just individual .dat files, then you can place them all in the Objdata folder within the Locomotion folder and they'll be there when you play the game next. If you have a .rar or .zip archive of the files, unzip them first before putting them in Objdata. If the mod uses an installer, then it will likely ask where you want them, in which case you should point to that same Objdata folder. You can tell when the game detects new files in the correct folder when the game says Checking object files as it loads.

Note that if the mod adds bridges, road/rail stations, signals, or anything else other than just vehicles, it will most likely need to overwrite a file in Objdata such as trackst.dat in order to work. If you have multiple mods like this, they may not all work properly together, in which case you may need to modify a file or two manually.

I did the said above and got my .dat files in the right folder and it said 'checking object files' but ingame, when in a world, the modded trains I got but they did not appear in the buy trains menu (i had installed modded trains).

I guess you put the files into the folder and immediately loaded a new game. Many people make this mistake.
You must first in the game in the editor and there select the vehicles you want, or deselect which should not be in the game. (Because note: You can only select 224 vehicles!)
Then save and THEN you can play this map which you have just created.

Adding onto what Walter said, you can do this for pre-existing maps, just save it as a new map and dont overwrite it. I wouldn't do it for Boulder Breakers though, as it can stuff up the tutorials from what I've found.