I don' t think this is a bug but more likely a conscious game design decision. I'm pretty sure the terrain and trees draw out to a maximum of 15km with the right settings. This seems to work fine for me in both TANE and TRS19. Limiting railcars and buildings to 4km doesn't seem that terrible a trade off to me. I'm pretty sure that railcars would be almost indistinguishable to the naked eye at that distance so limiting their draw distance to that doesn't seem unreasonable.
Larger buildings are certainly visible beyond this distance but unfortunately not all content is created equal, and unless all assets are well optimised and have LODs properly configured, rendering them all at longer distances could have a serious performance hit. Trainz already struggles in this regard.