No worries. As long as we all know what we're talking about we'll be able to help out.
If the tenon is sticking up in the air, think of it as the profile of a man. You have 'shoulders' on both sides (actually all the way around both the collar bone and shoulder blades are shoulders as well
) and a head sticking up in the air, on each side of the head are the 'cheeks'.
Sorta makes sense now, eh?
Your picture is what I termed side, meaning the side of a board flat.
This is illustration is making the 'shoulder' cut.
The cut I was referring to was with the board standing up.
That is making the 'cheek' cut.
And yes, a long bed rail is a dangerous thing to cut tenon cheeks upon. Lay them flat as shown in the above illustration.
Now short rails as used in cabinet doors for example would be fine positioned upright.
It is a learning process and we all sometimes attempt things we shouldn't when we first start out. If you mind says "this doesn't feel right"; stop and reconsider. Been there many times. Still find myself there today.