Bus stop mark is rendered bus icon in Mapnik when map is zoomed.
However, railway station mark is rendered to blue square.
Why? I think if railway station is rendered to train mark, It will be better to recognize railway station like bus stop in Mapnik map.
This is OSM Carto probably (default OSM.org map style), Mapnik is just a cartographic engine we use:
You can open the issue ticket here to discuss the problem you see: