Unwanted lines in endless 3D game

At first I thought you meant the yellow line and I was confused. But I take it you mean the lines at the end of each house. Looking around googele, it seems there's a couple of ways you can get rid of the lines.

If you go to scene view and select all your desired gameObjects(I'm assuming each house is a gameObject or something like that) Edit>Snap Settings > Snap All Assets. It will combine them all and get rid of the line.

But since you said this is an endless runner I don't think that's what you need.

So taking Jamora's answer from: http://answers.unity3d.com/questions/503574/can-objects-snap-together-in-game-without-a-grid.html

You could try:

You can use Physics.OverlapSphere on the newly instantiated cube to get all nearby other Units, then do a simple sort (example in GameObject.FindGameObjectsWithTag, middle one) to get the closest one. The actual snapping would be done by first parenting the new unit to the closest one, then adjusting the x,y position to begin at exactly where the other unit ends. Because you're using cubes, this can easily be done using renderer.bounds

Hope this helps!