The fact that she has gotten ahead of you and walked really fast for the train when the shift has ended a couple of times may have little to do with sending mixed signals and more to do with the fact that she just wants to get out of there.
It may have nothing to do with you.
Thereby, yes, there is a good chance that you are overthinking this.
She has stared and smiled at you often.
She has blushed a lot and even laughed at a joke you told that you believe that you ruined.
This is normally the body language of attraction.
It is also the kind of body language that indicates that she is comfortable with you and does not feel unsafe.
It is best to ask this girl out as soon as possible.
If you keep procrastinating on doing this, it may get to the point where she just starts looking at you as a work friend and nothing more, even if you were to eventually come around and ask her out.
It’ll also give you an opportunity to learn whether she’s actually in a relationship with someone else or not.
In essence, the sooner you ask her out, the sooner you will know if she is available or open to dating you.