The brick viaduct and the steel girder rail bridge have a lower maximum speed than it says in the tooltip, except on the start and end of the brigde.
The brick viaduct has 75 km/h instead of 100, the steel bridge has 200km/h instead of 250.
Maybe this has something to do with there only being 75 and 200km/h rails available?
The other 2 bridges are correct.
Playing on Simutrans 124.4.1 r11930
Please check again in 124.5, it seems to work there as advertized.