As briefly discussed in the Underground (http://forum.simutrans.com/index.php?topic=2730.msg84724#msg84724) thread, there is much to be said for changing the current ground-slope-walls-128.png image into something that looks rather like a brick retaining wall, as depicted, for example, here:
(http://www.ltmcollection.org/images/webmax/ce/i00009ce.jpg)
i guess, this graphic can't have time-depending versions, and also different versions in cities? I'm not sure if this wouldn't look a bit weird in 1980, and or on the countryside.
The current implementation is that all building foundations use a brick image and "ground" (including way) foundations use a rock image. Obviously you could just easily change it so that you always had the brick image. I don't think time dependent versions are currently allowed.
Maybe there is a chance to implement this retaining wall only to foundations next to railway tracks?
I have seen the brick foundation images for buildings - they do not really look like a brick retaining wall. It would be good to have a time dependant system (stone, brick, concrete), but that would take a fair bit of coding work, I should imagine.