Simply put, ship cores are ugly aesthetically and often a waste of valuable space on rail entities. What this suggestion entails is when you create a docked entity via advanced build mode, make it so that the Rail Docker placed functions as the reference point of that entity instead of a superfluous ship core.
I know what you're thinking, what about a way to enter and redock? Functionally speaking, not all docked entities will need to detatch and redock once they are in place, such as a landing gear. This will be the caveat of a core-less rail docker, you can only shift into them using the directional pad while controlling the parent ship. Should they become detached by accident, there's no way to enter them again to reattach if you jump out. I acknowledge that some would still want the current functionality in advanced build mode for docked entities that do need a core, so let there be a checkbox in advanced build mode to retain placing a core after a docker.
I know what you're thinking, what about a way to enter and redock? Functionally speaking, not all docked entities will need to detatch and redock once they are in place, such as a landing gear. This will be the caveat of a core-less rail docker, you can only shift into them using the directional pad while controlling the parent ship. Should they become detached by accident, there's no way to enter them again to reattach if you jump out. I acknowledge that some would still want the current functionality in advanced build mode for docked entities that do need a core, so let there be a checkbox in advanced build mode to retain placing a core after a docker.