this rout runs from Chicago IL to Los angles CA. I think it it is a big route tsw needs the trains are P-42 double decker
First off, yikes, you really need to edit your post. Second of all, DTG would have to get licensing from mulitple railroads to do the whole route, and if they did do the whole route it would be uimaginably expensive. I would suggest a section such as Chicago to Kansas City or Kansas City to La Plata. The "double decker(s)" are called Superliner (I) (II) Cars, by the way.
DTG_Luke and several others have stated there is not a limitation in the UE engine for route length. What has been limiting is the project time line as they build up their library of assets. Now what would be limiting would be simmers that would be willing to sit for hours as you cross the plains of Kansas, or high plains of New Mexico and Arizona. I live in the southwest and even though I enjoy the scenery, it would be a bit taxing for me to sit through in a simulation .