Can motorised, mechanised or armour units use these?
Yes they can, but the speed they can travel along a railway line is reduced by a % according to what kind of unit they are.
Infantry can use all bridges, but armour has restrictions according to how heavy the bridge can support.
Heavy armour can not use a light bridge for example.
Clicking on a bridge will tell what weight capacity it can handle.
Ferry's are a special kind of bridge, that share the same icon in game. that are actually boats, and take a long time to cross, once again restricted by weight limit.
Using the shortest route from the tool helper tab, for that specific unit,(lowest row of the three types) and clicking on the other side of the river by the bridge, will show you if it can cross it or not.