I did some gps tracing on Kallansillat in Kuopio, Finland. There are actually three parallell bridges crossing the lake Kallavesi. The left one is a bridge with combined bicycle/pedestrian resp. motorvehicle (southbound) lanes. The one in the middle is for northbound motorvehicles. And the right one is a railway bridge. Those all have a bascule mid section. Why isn’t the bascule section of the railway bridge shown on Mapnik? On Osmarender the bascule section is missing on all three bridges. A bug?
I bet most of all rendering programs doesn’t know how to render bridge=bascule. Mapnik is so kind to render everything bridge=* as a bridge. If you think the renderer needs improvement, post an enhancement request:
http://wiki.openstreetmap.org/wiki/Mapnik#Bugs_and_rendering_suggestions
Mapnik renders bridge= with any value for highways, but for railways it only renders bridge=yes or bridge=viaduct.
So the part of the railway tagged with bridge=bascule doesn’t render as a bridge.