Also,I only play singleplayer.I dont care about creating a server. Am I still gonna be able to add 3D models into my world?
Yes, basically everything that's available in multiplayer is available in singleplayer as well. The same applies to the API
Lastly,this locomotive model below is 2Mb and now I know it wont carry textures
This still looks like a high detail model.. Can you eventually provide a link to the model file? Then I can tell you more about it^^
It really depends on what you want to do exactly. For example if the loc has 100,000 triangles, that's quite a lot (I'd say it's "too much"), however, if you just want to use it in singleplayer and place 5 locs in your world and that's it, you won't have any problems at all.
One have to watch out to not exceed a certain triangle limit. Modern graphics cards can handle between 1 and 10 million triangles quite well.
How do I split the model into sections to add the texture in each one?
You have to do that in a 3D modelling program. For example 3D Studio Max, Cinema4D or Blender (which is available for free).