Thanks for posting that image, I dont see any brick facades on the "default" tunnels, meaning they are probably the Railtype Tunnel Base Sprites coming from OGFX-extra base-set (so not the OGFX-base base-set).
See here for a bit more information:https://newgrf-specs.tt-wiki.net/wiki/Action5
So they are Type 17/97
17 / 97 Tunnel overlay sprites
The sprites provide the ground for the tunnel sprites to be used by railtype NewGRFs which only need to provide the tunnel portals. They must be provided for both terrain types which could be used in any given climate (grass; grass+snow; grass+desert; grass).
What probably is happening is that you are using code from a railtype that uses this feature, so that they only needed to draw the facades. The rest of the tunnelsprite is then done via this feature (which is handy, saves a lot of extra drawing/coding).
If that is the case, try to contact the coders of that railtype to see how you can get rid of that feature.
If you coded it entirely yourself, you should be able to get rid of it by using an Action-5 (and replace those graphics with your own, or make them transparent), if you are coding in NFO.
However I think you code your railtypes in NML right? If so, you need the NML equivalent of Action-5, I dont know that one
In short, contact an experienced railtype coder, he/she should be able to help you out.
Shouldnt be a very difficult fix and then you can make your portals go deeper into the slope as you originally wanted.
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