They cost a lot of money and take a great deal of time to build. There really isn't a need for more than the existing transcontinental roads and RR lines.
Alaska.
The union had many trains, ships, and good roads. The Confederates had few trains, few ships, and not so good roads.
Pucca roads - roads paved with asphalt - are expensive to construct. They are constructed in areas with a lot of traffic. Many desert roads have little vehicle use so are not worth the expense and effort.
because the souther of Canada is cold
Automobiles are more convenient than trains for most people. One must get to the rail station and get there when the train is there. As train traffic declined, railroads ran fewer trains and serviced fewer towns. This lead to still greater decline, Passenger trains also compete with freight trains. If railroads make more money on freight, they were will slight the passenger service in favor of hauling freight.
Antisemitism and the Great Depression.
CaliforniaNevadaUtahWyomingColoradoNebraskaKansasMissouriIn Missouri it linked into the existing eastern railroad system (so it was not fully transcontinental by itself).
To support programs, education, health care, maintaining roads and highways and ect... Everything that you see in your community.
'The Last of Us' is not a Sandbox so to speak. Though there are a few back roads here and there to explore, but it is mostly linear.
It took me about 9 hours, driving in January on winter roads. During the spring and fall you can drive faster because snow and ice are not as much of a concern. Unfortunately it is usually slowest in the summer because the roads are so busy with traffic and so moving vehicles.
So few sightings are actually reported, there are no reliable statistics.