I haven't figured out a way in from there. The School District sealed up the bit that runs right underneath their building. You can see it on the bridge at Broad and Noble if you look hard enough.
However, you can wander on the old right-of-way between the tunnel at Broad and the Viaduct, which is now sealed off with a chain-link fence (phooey). It's easily accessible from Noble St.

Once upon a time, this line serviced Baldwin, which was located at Broad and Spring Garden, as well as the Inquirer. If anybody knows who used those two enormous Art Deco structures on the 1300 block of Noble, I would really love to know.

I've always thought they should turn the tunnel--more a cut, really--from Pennsylvania Avenue to the Viaduct into a greenway (with lots of lighting). That would spruce up the neighborhood some.

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This, this, this, and this are my new favorite things.

These two, not so much.

As far as I know the only way into the tunnel is at n30th & Pennsylvania ave and follow the tracks a couple of blocks till you reach the entrance. You can be under ground for a while until you come out at the police station at the other end.

There's a ramp between the Callowhill police station and the Whole Foods that wants to move to 1601 Vine. The police station's using the cut in front of the portal as a parking lot. There are also "No Trespassing" signs posted there so I don't know whether it's a good idea to go a-spelunking by that place...

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This, this, this, and this are my new favorite things.

These two, not so much.

I wouldn't ! There are police cameras watching !