Oops - that would have been me not paying attention when using Shift-R to copy bus route relation tags. Apologies for that (and thanks to TheSwavu for fixing up my blunder).

No - at this time there is no track installed there. When the Middleborough Road grade separation was built the space was left for a third track to be built to handle peak-period express trains through Laburnum - what could be called "active provision". For rendering purposes I would have used the tags railway=construction, construction=rail - to be honest they could be changed to railway=proposed, proposed=rail to keep the way in place until such time as the track does get built.