This tunnel under 6th Ave is known
http:// forgotten-ny.com/2015/09/under-6th-avenue/
But it does not run all the way to 52nd st.
However, the kind of tunnel that runs parallel to a street is usually built using cut and fill so, is it possible that there is more there but it was blocked up by some people who wanted it for private use. There is a subway station at Fifth Ave and 53rd street. Subway stations often have private tunnels behind locked doors.