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Pak128 / Re: Underwater Tunnels
However in the nighties there is a new feature that allows you to have slopes underground, you can download the newest nightly at http://simutrans-germany.com/~nightly/simutrans/index.php
All you have to do is build some underground track, then you can use the raise and lower slope/terrain tool (the same one that is used to change slopes above ground).
Just make an up slope or down slope on the last tile of track, and it should make a slope for you. After it makes a slope, you can continue laying track at the new level.