I wouldn't recommend this run; it's pretty cramped and hot. It's definitely not for beginners.
You can enter this run at the grate in front of Cassell. It's behind the big pine tree on the left side of the front of the building if you are standing with your back to Dietrick. The grate does not move quite as easily as some of the other grates, but it still works. It's a bit loud though, so make sure there's no one around.
After you climb down the ladder, you will find yourself in a pretty cool entrance room. There's a lot of graffiti in here. You'll want to be careful of the hot water coming from the machine behind the ladder, though. It could easily give you a nasty burn.
Continue on across Washington Street. It's a tight squeeze in some places, but it's going to be like that for the rest of the run, so just get used to it. After you cross the street, you will find yourself in an open area below all the grates and manholes across the street from Cassell. You'll want to take a right here; you have to squeeze by a valve to get by.
From here, the run is pretty much straight until you hit Barringer. You'll have to sidestep most of the way and crawl in a few places. Watch your head; there are a bunch of lights on the right side of the tunnel that you won't want to hit it on.
If you're observant, you will notice two inaccessible subtunnels running north on the left side of the tunnel. These lead to the crawlspaces of Pritchard and Lee, respectively. Unfortunately, they're no longer accessible because of the new pipes they installed in this run. After you pass the entrance to Lee's crawlspace, the tunnel will turn a bit, and then continue on straight.
Eventually, you will find yourself at a turn in the tunnel. Above you is the manhole near Barringer; it looks like they had to change the height of the manhole at one point, so there are actually two sets of ladders. It's pretty cool.
This is where the Barringer-Femoyer run begins; see that page for more information on the rest of the run.