"This is BloodyPenguin's Double Track Cargo Station ( https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/comments/703597323 ) used as both "sides" of a Cities:Skylines rail import/export hub. In this image, the regional/external rail system is to the right, and the city's internal rail system is to the left (*). The use of four one-way tracks, connected to the depot in this configuration, is the key to physically joining the two systems yet keeping the traffic segregated.

The inner tracks accept freight -- imports on the right and locally produced on the left -- and then the trains despawn, as they cannot travel farther in either direction.

Trains are spawned on the outer tracks to send the freight delivered to the depot (whether by truck or by rail) on its way, exports to the right and local deliveries to the left.

Edit (2018/02/11) --
Considering it a bit more, I think that with creative use of one way tracks "downstream" of the DTCS, it could be configured to send out both exports and local shipments in both directions, and to accept both exports and local shipments from both directions, while still keeping traffic from the two systems separate.


(*) Yes, the city's border is visible right there on the left -- at that time in the city's life, the purchased tiles are "L" shaped, so the internal network side crosses a bit of (currently) unclaimed land to get to the other leg of the "L", and thence to the Harbor district.


Edit (2020/11/17) --
There is a problem with this configuration, wherein trains will arrive at the depot and unload their cargoes (as expected). However, if they take on a replacement cargo, they will immediately despawn, as they are unable to exit the depot.

I have a suggested configuration which fixes that problem: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2289985525"