The Chattanooga situation is unique because of terrain. Mountains.
US-131 to I-67? AMEN!! An ideal situation would be a bypass for US-31 south of TC, joining an extended US-131/I-67, then meeting up with I-75 before the straits. (there are some that say the US-31 bypass south of TC is a waste, no doubt by those that have never driving through TC in the summer)
Not sure how it happened, but US-131 is a spur off US-31. US-31 is the "main" artery.
And yes, I agree, US-131 should go all the way to Indy, or at least connecting with I-69 near Anderson. I'm sure the state and local governments in and around Indy would cooperate with this idea.
Grand Rapids is indeed unique for the lower 48. It's is the only major city sitting on a major interstate (two digit) that never leaves the state.
As for light rail: trains are a lot more expensive to operate, and can hold many more passengers. To justify cost, I foresee something running every 30 minutes in GR. Just my opinion based on life as I see it.