Allow train stations to be built on the side of the rails as an upgrade.

If you where able to use this as an Upgrade feature, you would be able to use the "replace" feature, allowing you to build small part of a larger station, instead of having to build small station, when you need more, deconstruct rails. place new station wait to it to be built before trains can deliver.
Where the upgrade method would allow you to have trains using the rails until the moment it got all the resources and upgrades.