That bottom pic of the steam locomotive going over the water body, there is a crossing like that on the BNSF in North Dakota. There, the rail bed looks like big rocks thrown in. They were on it in a work train beefing it up last time I was over there (dumping in more rock). I notice the one you posted was all vegetation.
To model this feature, I would first pour a lake then simply either very coarse ballast or something that looks like large chunks of crushed rock (real rocks about 1/16" in size, and uniform in color, like crushed rock, would work). then use finer ballast right under the track, that would be very realistic.
There are lots of youtube videos about modelling water also we have an artist here (Jaz avalley) that sometimes posts has some great water feature tips. Here is a thread on it:
https://modelrailroadforums.com/forum/index.php?threads/water-and-the-modelling-thereof.35206/
May I suggest this water feature could be a good starting point. You will have to elevate the road bed leading up to it to get the feel right. Or, elevate the entire rail bed then you could get some relief opportunities.
For ground cover, where you might want dirt you can sift some dry sand to make it finer and glue that down. I have that kind of stuff all over my layout. There are other ground cover materials, the green crushed fine foam material is available from several vendors. Nothing wrong with putting something down then redoing it later, pretty much the story of my life (except marriage, you only want to do that once).
Dave LASM