Hiya Anes
The stairs mesh… https://playground.babylonjs.com/#KI8BJH#3 …is not easily animated in any realistic way, because it is a single mesh. We can position-animate the entire mesh… but the top/bottom stairs “landings” (the flatter areas at top/bottom of stairs)… will also move up/down the ramp… not looking correct.
#1 way: There IS a way to make this look/act correctly, but it will take some work. You would disable/remove the stairs mesh, and then build (in BJS scene) a top-landing, a bottom landing, and a single stair-step. Then we would “create-instance” that single stair step… about 20 times. Now we have about 20 single stair-steps, and we can apply a semi-complex animation “system”… to those 20 stair-steps. This is a substantial bit of work.
#2 way: We/Someone could “morph” the current full-stairs mesh… by moving/animating the mesh vertices (position points) of the full stairs… in a way that makes it look realistic. A guess… we would need to move about 240 position-vertices… every step of the animation/morph. This is also a fairly serious challenge.
I would say that method #1 is the wiser/easier way. The BJS ExtrudeShape should work, for making a single stair-step… like this: https://www.babylonjs-playground.com/#165IV6#821
I tried to do some instancing… to make many single-steps… but I failed. (Apparently I need to study more). 
Anyway, now you have more info, and a single stair-step. Others will likely comment soon… with more ideas… and maybe someone can get the instancing/cloning working for extruded mesh. And I’m sure that there are some “mad scientists” nearby… that would try (to get you started-with…) “The Escalator Animation Challenge”. Stay tuned.