Clip brushes wouldn't require 2 trains. The tracktrain entity allows them to by tied to it with normal brushes. But that's not the point. The train is supposed to have glass windows.
And making the windows larger does help a bit. However, when the train goes around a turn or gets trackswitched the glass gets a few degrees ahead into the turn, thus popping the windows sideways out of their frames. Maybe the train has to move at like 2 units per second for it to look right lol.