Here is the stat you requested:

“Overall, the system contains 248 miles (399 km) of routes,[11] translating into 665 miles (1,070 km) of revenue track[11] and a total of 850 miles (1,370 km) including non-revenue trackage.”

So revised:

NYC - 665 miles revenue track Seoul - 206 miles*

*A lot of sources report the Seoul number a lot higher, but include light rail and commuter rails - including those that go to rural areas. I’m comparing the core system (inside Seoul) to the NYC subway.

Indeed, in its reporting of ridership, Seoul includes commuter rails even to rural areas. So to compare ridership numbers with NYC, we would need to include the LIRR, Metro North, PATH and Jersey Transit