You need to provide some more information:

• what is a track number?
• how is it tagged?
• on what kind of feature is it tagged?

Preferably also link to an example where you think rendering this would be beneficial.

https://www.openstreetmap.org/#map=18/52.02950/8.53285&layers=O

That map shows various numbers in the area you linked to so could you please be somewhat more specific what tag you would like to see rendered on what kind of feature?

In German: „die Nummer des Gleises“ (the track number?)

It would be much easier if you'd just answer my questions. Possibilities what you might want to see rendered are:

1. ref on railway=rail ways - like 4000 here - this is not rendered on the public transport map but comes close to the concept of a track number. This is often ambiguous though due to different numbering systems.
2. railway:track_ref on railway=rail ways - like 301 here - same comment as for 1. above.
3. ref on route relations with route=track - like 4000 here - less ambiguous in semantics than 1. and 2.
4. ref or local_ref on railway=platform - like 2;3 here - this is actually shown in the public transport map but is a platform number and not a track number and - as the example shows - frequently contains semicolon separated lists.
5. ref or local_ref on public_transport=stop_position - like 3 here - this is probably one of the numbers that is rendered in the public transport map - the numbering of the stop positions however does not necessarily have to match that of the tracks.

I think „5.“: My question: my itinerary shows me: change trains in city X. I arrive at platform 3 and am supposed to take the onward train from platform 23. My idea: as a traveler I open OSM (in cityard mode) and . . . I don't see any track numbers . . . So I have to orient myself on the platform and can not prepare.

@imagico: Thank you for the feedback and the effort you have made

I have changed the issue title to more accurately reflect the request.

I don't really see how this can be implemented at the moment but i am no expert in public transport mapping. From a quick look it seems the vast majority of public_transport=stop_position are at bus/tram/train halts where there is no locally differentiating numbering (i.e. where there is no number or the number is a trivial '1' with no '2' or other number being nearby).

Someone would need to analyze the mapping practice world wide to determine if there is a reasonable way to label the meaningful information without filling the map with a lot of pointless labels.

For public_transport=stop_position the means of transport are specified with e.g. train=yes, subway=yes, tram=yes, or bus=yes. This way one can limit the rendering to trains (Overpass querry: https://overpass-turbo.eu/s/1sZ2).

I would prefer rendering ref, local_ref appears to be a "misused" tag (1).

(1) https://wiki.openstreetmap.org/wiki/Key:local_ref

I opened a pull request for this (#4800)

Open Questions:

• should both ref and local_ref be rendered or just one of them?
• I observed some rendering artifacts between tiles. Can one avoid that some way?

• Both ref and local_ref are widely used so I'd render both
• The rendering artifacts are not caused by this pullrequest but by the docker setup (see Labels cut off when using Docker #4808)