.. Dealing with the tentacle

With tools you use when dealing with other jams in the piping systems.
Have you entered the train, already?

Good. Then you've must found the darkside through the portal.

Did you spot there's the same rooms and areas than on the brightside? ( There's also toilets. You just need to get past Chief Ghouley. )

Visit the toilets to find a plunger. ( Go check the toilet in the last stall. )

Great! It doesn't work alone. You need a handle for it. ( There is a plunger handle in the janitor cart at the brightside platform. )

Now combine the handle and the plunger head to make a plunger. You can now push the tentacle into the toilet.

Any guesses?

The tentacle is now on the darkside middle toilet stall. By using the plunger there you could send it back to the brightside. Neat!

Exactly. There will be an item you would like to send to the brightside. Remember Purple, then.

The control room key you find from the darkside control room key rack. You can pass it to the brightside by giving it to the tentacle.