Your argument, as I understand it, is this:
(premise) When a light rail line is built it causes a decrease in traffic congestion due to motorists choosing to ride the train vs driving their own vehicle.
(premise) This decongestion benefit is negated by new motorists choosing to drive on the less-congested roads, thus making them congested again.
If I missed something I don't care. I'm moving on. You have not clearly communicated your point such that I inferred you were arguing the opposite of what you thought you were arguing. If you can't or won't communicate an idea or argument clearly it doesn't matter how sound, valid, or true your intended argument may be. You may be correct but being a smug dick about it makes you more wrong than right.
The two premises are taken directly from Induced Demand Theory. The subject is broad and complex, but there is enough evidence to support the premises as correct facts in this argument. However it should be noted that the evidence is not conclusive, so taking the two premises as absolute facts is not appropriate.
Don't celebrate yet. While your facts are factual and they support your conclusion, that does not mean your argument is sound.
By the strict rules of an introductory college logic course, yes your argument is valid. But a valid argument isn't also a true argument.
What I am stating is that 1) your conclusion is too broad and 2) it grossly simplifies the complex issue of 'are light rail lines a sound investment for the MSP metropolitan area?'
A new light rail line may not conclusively nor immediately ease congestion but to dismiss light rail entirely ignores its other benefits. The most explicit benefit backed by lots of research is that a new line causes property value to go up. I'm stating that as a positive, but I recognize that for some people and situations that is not always the case.
A new light rail line adds to the economy as more people are living and shopping within an area. Sure congestion may have stayed the same, but more people are living within and using local neighborhoods.
Light rail, same as for other public transit, is disproportionally used by lower income persons. It enables these people to have greater mobility. It's kind of a way to spread out wealth across a region which helps people from all economic classes and can have a more profound effect on lower income persons.
To me, what this whole comment chain is about is whether or not MSP should add to the light rail system. The MSP area is expected to grow in population for the foreseeable future, so it seems congestion is just something we will have to learn to live with.
Regardless, this is will be an on-going discussion that Minnesotans will be having. Communicating a fact in a smug and poor manner does not help that discussion.