Bonjour Andrew,

This type of query has been asked many times on the forum.

I cannot improve on the lesson in terms of explanation.

"Descendu les escaliers et descendu du train"

When the subject employs the verb descendre directly on the object (avoir usage -- transitive)

the effect is that the subject is doing something directly to the object.

This could mean "to go further down" or "take down" or "put further down"  depending on the context.

So to "descendu les escaliers" could mean dismantling the escalier(s) or lower the escalier(s) physically (manually as an operator of the escalier) 

But when the subject employs the verb descendre via a preposition (de le (du)) then this means  

that the subject is coming down from something (a train or aircraft) or getting out of something (a car) the key being the effect of the preposition between the verb and the object ( être --  intransitive usage).

Hope this helps somewhat 

Bonne continuation

Jim