In Simutrans, there are various ways of crossing the sea, though they
basically boil down to bridges and tunnels.
Bridges can be built over any depth of water, but need a straight
slope of land at each end. Remember that clicking on a valid slope
will build a bridge (or tunnel) all the way to the far side.
Elevated track (Brick Viaduct Elevated Railway for railways, Elevated
Track for maglevs) can be built on shallow water and doesn't need
anything special at the end; in the case of maglevs, you can build the
elevated section in isolation, then throw a bridge out to it from the
shore, and the bridge can cross deeper water. In any case, use a
ctrl-clicked bridge segment to come ashore at grade. (Yes, you can
build a rail/maglev bridge out to your oil rig or fishing ground if
they're in or adjacent to shallow water. This will very rarely be
cost-effective, and there may not be suitable rolling stock available,
but it is silly.)
Tunnels cost no more than bridges, and don't have the same constraints
about running in orthogonal lines.
Start with track running at one elevation above sea level (typically
-1), and two empty squares in line with it. (More generally, start
with a 3×2 flat area with one more flat in the direction of the
tunnel.
Lower the two corners farthest from it.
Place track onto those two squares.
Place (ctrl-click) a tunnel onto the farthest square.
Use Slice View, and go down one level (-1).
Artificial Slope Lower the square inside the tunnel. (It will disappear.)
Move the view down one level.
Build more tunnel.
If you want the tunnel to be deeper, Artificial Slope Lower the tunnel
square furthest in, then go back to "Move the view down one level" and
repeat.
Otherwise, continue the tunnel at the current level.
Note that this digging technique doesn't allow you to tunnel up.
You'll need to dig down at the other end and make the two meet.
If building a completely enclosed system, you can finish by removing
the original tunnel mouth and tracks and restoring the flat ground
where you started.
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