Historically prominent economist Anthony Downs books "Stuck in Traffic" and "Still Stuck in Traffic" provided the best explanation of rebound that's in print.
The basic notion is that for every ten commuters who switch from driving to the train creating a little more space on the highway, there are ten other commuters who in response find driving more convenient and who will switch to driving in those ten empty spaces. Where do those ten others come from -- (1) those on other, slower roads; (2) those who ride transit, but would switch back to driving if the congestion dropped a little; (3) those who leave early because of traffic, but who will start leaving later if peak traffic eases; (4) those who are telecommuting because of traffic, but would rather go into the office if there were more space on the road, and so on and so on. Consider that modern commuters have smartphone apps that will tell them if the traffic is showing any sign of easing in their favorite commute hour, and they will jump deeper into traffic once they see relief is possible. A few people in some cities choose a different route every day depending on the traffic that is showing.
Of course, there are some people who don't like driving and will take the train even if traffic were to get a little better. The ridership of transit in Seattle is generated by those people, who would really prefer to ride transit than drive. Bless them. One problem is the basic statistical fact for the vast majority of commutes in Seattle and other places in America, is that the commuter who compares door-to-door travel times for commuting by transit, or by private car, for the majority of comparisons, even with traffic, will find driving is faster. Another factor is that the wealthy commuters can buy or lease a car that drives itself with automation in stop and go traffic on freeways ... available for purchase, right now.
What does work for pushing people from driving to taking transit is putting new peak hour tolls on more roads, and/or raising the price or inconvenience of easily parking at work places.
Spending billions and billions on more light rail in Seattle region will not reduce traffic on the parallel roads, even though tens of thousands may decide they will ride the trains. Millions will still drive.
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