Steve;
As you have said, this is a first step. I've made many of those! These comments are just MHO.
As drawn, I initially thought for a first go it was fine, but after studying it for a while, I saw these potential problems. I hope that this critique helps and not hurt.
First off the biggest problem I see is that you actually have only 1 track in the main yard, that can function as a yard track. Plus this track is very short. The rest of the "yard" tracks either serves as a mianline or services the roundhouse, ashpit, TT & coaling tower. Using any of the other tracks as part of the yard will block either the main or access to the roundhouse and its associated tracks.
There is a "bypass" track that runs from one end of the right loop, "through" the yard, and reconnects to the main. This track turns the end loop into a reversing loop, which turns the train back onto the track it came from. If you're going to use DCC, wiring is simple, use an auto-reverser on this track to change the polarity. If it's DC, then there is some special wiring that has to be done.
As drawn the section to the left of the station, appears to be a diamond crossing and not two switches back to back. If this is a
switch arrangement this causes another reverse loop. This will make the wiring even more complicated if the layout is to be DC.
As it is drawn now, there are no passing sidings. This means that you can only run one train at a time. There is no way that two trains starting in different directions can meet or pass each other.
Lastly, since the engine terminal appears to be the main focus of the layout, the yard, drawn as is, can't support such an elaborate terminal without there being more tracks in the yard, even if the purpose would be to bring in a train and change the power on it. without fouling the main.
Suggestions;
Redraw the right hand loop to come out on the opposite side of the turntable, and move the station some toward the backdrop. I would also move the yard "throat", (the beginning of the yard in this case), a little more to the left. This will allow you to draw in more "true" yard tracks and makes the engine terminal look more like it belongs there.
Draw in a least one siding. The old rule of thumb is you can run one more train than there are sidings. If you have one siding, you can run two trains, 2 sidings 3, 3 sidings 4 trains, etc...
Definitely include more spurs for picking up and dropping off cars.
I do urge you, if you haven't already, to get and study John Armstrong's,
"Track Planning for Realistic Operation" and for you to read online;
Our own
Spacemouse's beginners guide at here;
http://www.chipengelmann.com/trains/Beginner/BeginnersGuide01.html
or this one from the NMRA;
http://www.nmra.org/beginner/
As you said this is a first step and while artistically it looks like a big step, unfortunately, the functionality turns it into a very small one.