I use to watch trains here all the time in the late 90's, early 2000's. Use to be stand right above the mouth of the tunnel back then. Fantastic view point! Haven't been there in years, not since all the new security. I have no idea how close you can get now.
It’s just a stairway up towards ground level. The actual slope of the tunnel starts a few hundred feet away, so unless you want to walk it or jump down 30+ feet the best way to the tunnel entrance is the stairs ;)