I was thinking it was being pushed.
Typically this configuration does not have any actual “pushing” going on. The tail end engine is either just along for the ride and for the convenience of changing direction, or it is providing much less power than the lead locomotive, so that it does not “push” harder than the lead engine is pulling. The main motive force is always the “pull” of the lead engine.
You could have the rear engine doing nothing but braking, that’s stable.
You could have the rear engine provide 50% of the power of the front engine, just helping. That’s stable. The rear engine is just making the pull easier.
You could have the rear engine provide the same power as the lead engine. That’s very tricky to maintain stability. If the front engine slips and the rear doesn’t, you have a problem.
You could have the rear engine provide more power than the front engine. That would be instantly unstable, of course.
There are several different ways to arrange the locomotives and use their power to move the train. You commonly see helper engines in the middle of the consist, controlled remotely by the lead engine.