I find it quite inconsistent that there is a separate tag for indoor corridors (highway=corridor), but no separate tag for indoor steps.
I think it would be a good idea to invent a new tag. E.g. highway=staircase.
What do you think?
(This question is related to the Indoor Navigatum import in which we want to import several indoor steps / staircases)
As I noted, most highway=corridor already have indoor=yes for some reason. Deprecating it, and moving to highway= + indoor=yes would be better. There are already 28k =steps + indoor=yes Tag:highway=corridor - OpenStreetMap Wiki
Eg highway=elevator + indoor=yes can be used for inclined lifts indoors. Indeed there are 56 way[highway=elevator][indoor=yes](if: is_closed()==0); among the 349 unclosed =elevator lines. For comparison, node[highway=elevator][indoor=yes] is 6.7k making up 16% =elevator now.
at least some indoor steps are not staircases at all - at least I assume that say 3 steps appearing somewhere would not be called staircase
It’s the other way around: highway=corridor was always a mistake and there is no reason to repeat it
Most data consumers would probably like to hide corridors and staircases inside buildings but show highways inside underground stations. So it would be good have something that distinguishes between these two cases. The footway / corridor distinction currently provides this distinction with ways in underground stations often tagged as highway=footway + indoor=yes and ways inside buildings tagged as highway=corridor (+ indoor=yes)
But I can also understand you point as the line between footway and corridor is very blurry.
That wasn’t the point though, SIT already has the indoor=corridor element to model corridors, and if necessary something similar could be defined for staircases (if it doesn’t already exist).
But indoor=corridor is only meant to be used on areas and I have never seen it on a way so far. Or are you suggesting to also model the staircase as an area? - That currently seems like the best solution to me as it would fit well into SIT and avoids having to map both area and line all the time.
Some links that might be useful:
In short, there’s a bit of everything. I’m not saying that any of the above are “correct”, merely that they are in OSM 
It was always the intention that indoor=corridor would be mapped as an area, and that either area routing or routing along the outline should be used for routing applications.
I think this is a viable solution, if there is a staircase. When the stairs are a crossing element it could be confusing to map the area, e.g. here
But why would it be confusing to map these stairs as areas?
For me the difficult part for areas is rather how to tag which way leads upwards as you can’t use incline=up/down. The best I could come up with so far is something like setps:orientation=along/across or steps:direction=N (borrowed from roof:orientation and roof:direction). But setps:orientation doesn’t give you where the top and the bottom is and steps:direction doesn’t work for curved staircases.
Overlapping features doesn’t work in lines either. Drawing highway=steps as an inward spiral close to each other is only a workaround.
For non-overlapping curved stairs, in indoor= , ultimately it’s possible to determine the direction from the level= it connects between. So it’s not absolute necessary.
An explicit solution could also be a point, instead of a line. Although, lines are standard in 3D modeling/CAD, and already done for area:highway= in OSM.
Did the photographer have inspiration from M.C. Escher ;?
/OT
