Don't know how it is in the US, but it's similar for cars, highways don't cross small routes either there's typically a (small) bridge involve (no need for viaduct).
Limited access, freeways in crowded areas take space, hence my point about viaducting (or tunneling) to create more space. But ya, if you pay for the spaces of the bridges over the freeway through your downtown urban area, you can also do that with a train if you have more space for it (or tunnel the train under). China just doesn’t have that kind of space, which is why you see viaducts everywhere in cities like Shanghai or Beijing.
