City Life

Certainly there are lots of cases of plain-bad road designs causing unnecessary bottlenecks. And at least as you have described it, this sounds like a bottleneck. Congestion can either be blamed by plain old demand or by such bottlenecks. From a traffic engineer's view, demand-based congestion is a good thing because it means the road is highly utilized and working as intended. And bottleneck-based congestion is just negligence.

That said, increasing road capacity is a one-way ratchet in exactly the way you have just described. This gradual widening of roads just continues moving congestion around the network to other places until, the engineers will say, it's diluted enough that no one notices it anymore. At some point, a road was widened or a new spur connected or other forms of growth happened that made this 2-lane section inadequate when it once was probably more than enough. And fixing this road WILL make the place you live suddenly more appealing to, for example, a commuter student who couldn't previously tolerate the long commute. Long-term, it will inspire car-dependent growth. A new choke-point will emerge. Either that or the region simply stops growing -- which is a different kind of death sentence with the way modern society is structured. You can play whack-a-mole with the choke-points or you can invest in an actual solution like, as you correctly pointed out, adding the train service

Adding that train service does the same thing, by the way. It inspires growth towards train-dependent development. The difference being that trains mostly become more economical with scale while cars mostly become less economical with scale. Induced demand is a law of nature, at least so long as the population and economy are growing. Which we have to assume until leaders are willing to swallow how apocalypticly-dangerous it is to found all societies on the debt/growth cycle rather than sustainability.