In case you didn't see the reply of raymorris below, your guess that "you", the "train passenger", pays for infrastructure for motorists is quite in fact blatantly wrong and actually the inverse; motorists pay a portion of the infrastructure costs of a "train passenger."
Also I second what raymorris says; the problem of traffic jams is factors such as accidents. I would add that, related to Chicagoland, my rant:
Policemen sitting on the side of the road causes traffic to slow down dangerously and abruptly when 'stupid' drivers see the police officer (since everyone is 'speeding' above the 55mph) and think that by braking they are going to somehow help the situation; in fact it causes a chain reaction of brakeing aka traffic. On this topic I think it would be great public policy of Policemen weren't allowed to sit on the side of the road acting like they're going to pull someone over for speeding; they never do! What are they doing? Waiting for an accident to happen so they can go respond? They're really raising the probability of an accident it seems by creating traffic.
On that topic 90% of drivers think they are in the top 50% of skilled drivers, and that drivers who don't drive like them are 'wrong' and 'stupid', including myself. So this means when you get the idiot incapable of managing a short but safe distance, breaking irregularly, or enforcing a 60mph speed in a 55mph zone because "people who break the law by speeding are stupid and I need to do something about it", this also causes traffic. I follow cars extremely close and drive 70+mph like a majority of drivers in rush hour Chicago, and it works great as long as everyone else maintains the pattern; counting on the fact that the driver 4 cars ahead of you braking will cause the car 3 cars ahead of you to need to brake, etc., but not really caring about the driver directly ahead of you because for all intents and purposes the driver ahead of you should be driving and not braking on the interstate highway system.
I would gladly give up my extremely-close-following-speeding driving pattern when we all have cars driven by AIs and the cars are almost touching bumper to bumper in a much more optimal configuration for high density traffic, but for now I'll just have to continue cursing at the idiots who actually think they are helping by being the standout fool braking and leaving too much space. Thanks for the traffic today, stupid person!