java - A queue of background tasks in rxjava

Is it possible to implement a queue of background tasks using rxjava?

I need possibility to add task at any time and only one tasks executed simultaneously. I have tried PublishSubject, but when i'm pushing execution to the new thread observeOn(Schedulers.newThread()) it starts more than one task at time.

UPDATE:
Is it possible to implement something like producer-consumer pattern using rxjava?