its not standard U.S gauge. meaning nyc subway trains or any other kind of trains can run on it.
Before we get back on topic, I just want to point out to you an error in that statement.
A different gauge means that the distance between the two main rails that the train runs on are different. It's impossible for every other car to run on that gauge if those cars are not built to run on that specific gauge. The Market-Frankfort line uses a broad gauge, meaning that its two rails are 5 feet and 2.5 inches apart. Standard gauge means that the two rails are set 4 feet 8.5 inches apart. Because the distance between the wheels of a train cannot change sizes, trains cannot run on broad gauge if they were specifically made to run on standard gauge.