When browsing the forums, i noticed that in our few games so far, we have been a bit too liberal with tile upgrades, only checking if the company upgrading has a legal route on the hex, not route to the new segment.
That's the rule used in some games, for example 1830. 1870 has a more restricitve rule.
But the rule: "must have a legal train route of any length from one of it station markers to and along one of the new
track segments" ->
by a company, that doesnt have a legal connection to the original straight track, but has a connection to the new track passing over?
Yes. That's what the rules says. You need a connection to the new track on the bridge.
On the other hand, a company that has only access to the straight track on the original tile can not do this upgrade.