Rail and subway are distinct modes.
Many subway-trains in Japan are trains for the first 20 stations, then "subways" for the next 10, then trains for the 20 stations after that. Same cars. They may have been 3 separate lines at one time but they got connected.
Also, from a regulatory standpoint, freight and intercity railways (e.g. not subway/light rail/metro/trams) are subject to full FRA regulations, compliance with which would add even more costs, for little real benefit at the speeds they travel at.
Plus, in practice subway/light rail is electrified, and anything else (again with few caveats) is not. In towns with both it is very rare for them to use the same stations.
*There’s probably an exception to the exception for the Akita / Yamagata branches that partially run on regular train lines.
There are few rail networks that manage to serve both well.
Yes, and it's terrible. The Montreal subway is by far the noisiest subway system I've ever used. Even standing on the platform is a terrible experience because it's so loud when the cars are moving.
Both seem to come in around 80dBA average, but some of the tube lines peak at 90 or 100+dBA.
But that's not really what I'm talking about with Montreal: in Montreal, the subway system is loud for the people standing inside the station. (The trains are noisy inside too, but that's just in addition.) Every time a train enters or leaves the station, it's deafeningly loud, because of the rubber ties. You'd think it shouldn't be this way (after all, cars have rubber tires and are very quiet at low speeds; it's only at freeway speeds that rubber tires get loud on cars and trucks), but it is. It seems to be caused by the trackways: the tires are constantly rubbing on the sides of the track, so the cars are very noisy any time they're in motion, and it's really miserable just standing on the platform.