you can move thing with double G key accuratly as 0.01
what is the monrail mod?
is it a workshop vehciule or a hacked .pak file ?

It's Zubenko Mehali monorail mod from workshop

I tried the G key but it doesn't work. When can you use it? While spawning the object?

Slow your freecam down to a crawl and spawn pinned, keep flying around and respawning till you get it like you want it. You may also need to make sure that the zoom is all the way out depending on the size of the item being placed.

ah ok i get your question
you want spawn on rail so you just have to use the spawn menu of the gare itself
i tought you was talking of the editor for the G key
and learn than a mod is never in the workshop even if its the name of this item
i guess its a modded version of another monorail
and not a mod for the game
perso i never reached to spzwn things in free cam

How to select the entire vechicle/track in the editor?

Nothing selected, invert selection.

you can also select bricks per cathegory or by contigousity
just hold the esc key will open the menu at right side of the screen and in the tab selection you can click and learn the key and eventuially reasign them with the crank icon
perso i use exclucively the L key for select contigous brick and also the ctrl L for all contigous chain

How to rotate vechicle without it splitting apart

Everything must be connected to everything else and connected things highlight green, toggle snapping on.