HOW CAN I IMPROVE MY MAPS ?

You can just add a wall with stairs leading down behind them. That way it looks like you're heading down without actually having to make all the steps and railings. Or you could make something like so:

Mind you, this is pretty roughly done, but I hope it gives you the idea.

Both of these maps are really gigantic and bare, maybe reduce the empy spaces somewhat and add a bit of furnishing.

They look fine to me, am I missing something?

You could have plants but also notices on the walls, or boards with informations, coat racks. Ah, should those ceilings be so large on each side and not just follow , one tile, the configuration of the space? Otherwise, looks good.

You should probably put in some ticket barriers so that people can't just run up to the trains and jump on board without a ticket, or while the train is flying past. A security booth or two might be nice as well.
I know of some graphics coming out soon that might fit the bill, so if you're patient...
But yes, ticket/timetable boards, food stands, bins. Also, why use the grey fence up top instead of the wood? Use either all wood or all stone else it looks very odd.

Right, that's perfect. I also prefer the yellow fence and the second map, that big grey bar doesn't look great, or the first map, because in that one those signs are where they're supposed to be, but if you take out that big grey bar above them.

Edit : oh about the school ceiling, just a detail, you forgot to take out the part along the extreme upper right.

The image is a bit too small to fit. Look for something that's larger than the screen size then cut it down or put it in as is. You can tell it's too small thanks to the reset at the bottom of the screen.
Otherwise, I guess it looks pretty decent. ^.^

I'd say it's fine.