You can simply use your browser's web developer tools to see which URL is behind which layer. Most layers are served from a different tile server. Currently these are:

 - Standard: tile.openstreetmap.org/
 - CyclOSM: tile-cyclosm.openstreetmap.fr/cyclosm/
 - Cycle Map: tile.thunderforest.com/cycle/
 - Transport Map: tile.thunderforest.com/transport/
 - ÖPNVKarte: tileserver.memomaps.de/tilegen/
 - Humanitarian: openstreetmap.fr/hot/

The [Tiles][1] wiki page is just an example list of tile servers. It can be edited by anybody, so feel free to add the Transport layer to this list.


  [1]: https://wiki.openstreetmap.org/wiki/Tiles#Servers