Max Semenik adds,

"the problem here is that e.g. google cheats and shows all the tracks amalgamated into a couple lines, which they can afford to draw thick

• https://en.wikivoyage.org/wiki/Tokyo/Roppongi#/map/0/16/35.6597/139.7489
• https://www.google.com/maps/@35.6581863,139.7511662,16.02z

Maybe subway/metro/tram/light rail/train/monorail/streetcar services could be shown with their colours (fromthe OSM relation's colour/color value, if available) in a separate layer to the normal map (hosted on WMF servers)? I guess the colouring could be a bit difficult for multiple services sharing one track, but multicolour dashes could be rendered.

(This comment originally referred to adding a layer, but not being a frequent Wikivoyage user I didn't realise that there was already an openptmap.org overlay.)

One option which is already available, is to show train lines using "ExternalData" (geoshapes) capability - https://www.mediawiki.org/wiki/Help:Extension:Kartographer#GeoShapes_external_data (see the highway example) - assuming that OSM has marked local train lines with some Wikidata ID. I don't know if bus lines are marked in OSM, but they can always be drawn using GeoJSON directly in the map. I hope we will provide a way to share data between pages fairly soon.

I guess this would work for Wikipedia (in articles about an individual system or a single bus route), but not Wikivoyage (just a layer or layers for the entire world).