Slitherine

I haven't had time to check, but is there any time a unit can be on a river hex and not count as on river?

I had a tank just attack an infantry unit who I assumed would be in the open as they were on a rail bridge over a river, however looking at the logs I actually got the full +4 river bonus for my attack and defense.

Yes as far i can tell bridges counts fully as river hexes. I don't know if this is an oversight or intentional but it seems to be a bit odd that using a bridge applies same penalty as fording the river. Obviously a bridge is by far not the perfect hex for a battle but still better then fording a river. An own terrain type bridge could be helpful here.

Just guessing but I doudt its a design oversite, considering the scale of the game where a unit might be considered a division and a hex 10k or so. Since rivers are passable, although it takes a full move to do so, i imagine that the unit is slowly fording the river, using smaller bridges, shoring up/strengthening others for heavy euipment/laying pontoons etc, hence the long delay to get over. Bridges might be several large bridges that dont inhibit movement but still are choke points really easy to defend.

Just guessing but I doudt its a design oversite, considering the scale of the game where a unit might be considered a division and a hex 10k or so. Since rivers are passable, although it takes a full move to do so, i imagine that the unit is slowly fording the river, using smaller bridges, shoring up/strengthening others for heavy euipment/laying pontoons etc, hence the long delay to get over. Bridges might be several large bridges that dont inhibit movement but still are choke points really easy to defend.

Correct, consider a 10 km hex, your tanks approaching, fighting their way to the river, fight their way over the river and repulse a counter-attack than form a bridgehead. It is not a tank on a bridge.

This counts for all battle, abstract the hex that when battle begins you engage the edge of a front and have to fight in that 10 km zone with your regiment/battalion/corps.