There is a keybox at the upper level.

Well, now, it is a keybox.

There is a key hanging on the hanging skeleton at the Viaduct.

Luckily (not for the skeleton) the skeleton is hanging on a semaphore that can move up and down.

There is a Lever on the Upper Level that you can go and push. Notice the lights changing.

Go and get the low hanging key and open the keybox. You get a handy wrench!

You can use it to open maintenance hatches.