This is a question of logic rather than mere proofreading.
All statements start in the same way with
{Statement about an event at a past time}
Ten years ago he showed me the location of the stations.
This is followed by three alternatives:
The bridge is known to exist now
{Statement about a present truth relating to those stations now}
There is a bridge that connects them.
This tells us the bridge exists now but does not tell us if it existed ten years ago or not. To suggest that did not exist ten years ago you could say
There is now a bridge that connects them.
Emphasis using "now" suggests a contrast with the past, implying that the bridge did not exist then.
The Bridge is known not to exist now
{Statement about a past truth relating to those stations at that past time}
There was a bridge that connected them.
Although true, this does not tell us if the bridge exists now or not. To suggest that it no longer exists you could say
There was a bridge at that time that connected them.
Emphasis using "at that time" suggests a contrast with the present, implying that the bridge no longer exists.
It is not known if the bridge now exists or not
{Statement about a truth relating to those stations at that past time}
There was a bridge that connected them.
The last statement tells us that there was a bridge in the past but tells us nothing about the present. To suggest the uncertainty of its still being there - or if it still exists or not - needs additional prose. To give only one example: There was a bridge, which may still be there, that connected them.