Create: Mail Rail

Adds a new action to the train schedule that:

* Acts identical to the standard "Retrieve Package"
* But excludes any stations that have something headed to them already

This prevents the clown car situation for wild-carded (or just a lot of) trains at a central location, where they all head over to get a package at once, even if it's just the one. as soon as the station's free, other trains can head to it again

This unfortunately does mean that if you have enough packages to fill up a train (unlikely), there won't be multiple trains to collect, but the original retrieve package action is still there, and is untouched by this addon.