Sorry for the late reply, I get a notification when a comment is made but not when a discussion thread is started.

If you have both ends of each station connected then they should work as designed, I can't see one of the entry points where you have the sloped track coming out of the tunnel and connecting to the station.

If both ends of each station are connected then the only reason I can think of for why the traffic is only importing is if your cities exports have a more efficient route elsewhere which the AI is prioritising.

Interesting layout having tunnels coming into your cargo area, obviously you're wanting to avoid level crossings which do create traffic issues.

There are a couple of ground level rail terminals which use the same footprint as these terminals, I used them as a template for my elevated version. They may fit your layout better. I designed these terminals to work with an elevated network.

Thanks for the reply. No problem on the wait.

These are two end stations placed side by side. One for outside connection cargo and one for inside connection cargo. Roads were connected to both stations allowing flow in and out of each. But yeah, trains never spawned from the station and replacing them with normal vanilla station resulted in cargo trains spawning as expected so I don't believe it was a case of "more efficient route"

I'm still subbed but haven't been using these stations. I was thinking about trying the same idea with the side entrance station but have not gotten around to it yet.

Looking a bit closer at your screenshot, do you have one way highway roads connecting to the visible side of the station?

Cargo trucks do have strange behaviour in all terminals depending on how close roads are to rail. Having a different road type may cause problems even if there isn't a logical reason why it would. Other than that and the sloping rails leaving the station I can't see anything different to how I would set up these stations.

Sloping the rail down straight away is something I have never tested as I designed these to be used on an elevated network. Bizarre as it sounds, having the rail staying at the elevated level until it leaves the immediate area of the station may fix it.

It was a 1-way system into one side of the stations and out the other but I had 2-way roads connecting the stations to each other to allow trucks to cycle between them without getting into the rest of the industry area. Reading up on the issue Lost Nomad was having and Enma's comment, its sounds like what I was doing was similar so I may give it a go in the future.

Jumping in on this discussion...
I found the idea neat to use these stations together with the harbour in the same design to build a modular hub (as they were intended).
After a bit of fiddling I found a way to place them at the exact same height to match up and give the illusion of a single large building since stations and harbours tend to break when touching them with the Move It mod.

I replaced the road inside the harbour with the 2-lane oneway of same size and attached two through stations, leaving their roads untouched. The one adjacent to the harbour never receives trains but is able to spawn. Same behaviour with the terminus module. The second station placed always works fine and as expected.

I tried placing the stations 180° flipped because I thought of spawn points and roads interfering with each other too, like you implicitly suggested but that didn't change the behaviour of any of them.

Not asking for a "fix" because I suppose it's the game's AI being rude to me and that usually requires more hassle than it's worth to fix it, but just wanted to let you know.
I'll be deleting the "middle", i.e. the station adjacent to the harbour, and filling the gap with a PO to maintain a coherent look. It would be really grand though if you would make a "dummy module" or maybe a small warehouse in the same style to make filling that gap more convenient in a similar spirit as your modular freight warehouse terminal.

The cargo truck AI is frustrating. It's hard to get them to work perfectly. I like the idea of a warehouse only version. I'll see what I can sort out and will get one on the workshop soon.