One of the things I'd really like to see added is some sort of postal items system where we can load and unload mail onto trains and transfer the mail to other locations. There's general support to do something like this via books (mail) and shulker boxes (packages) but I'd love to see a little flavor added for it since trains are now a thing. If this is too much out of the scope of Create, that's totally fine, perhaps I'll see if I can add it as an addon mod for Create instead.

You could try these mods, especially Snail Mail. It has a similar style to Create.
Snail Mail
Ender Mail

You could also set up a mail system using Create's preexisting systems. Filters may be used to unload trains containing signed books.