In my classes on wheel/rail interface, one of the first things I challenged my students with (experienced railcar maintenance people) was "Can anyone tell me where the STEERING WHEEL is on this train?"
They'd all laugh.
I'd repeat it.
They'd kinda chuckle.
I'd repeat it again...
They'd look at me like I was nuts.
EVERY WHEEL is a steering wheel.
What keeps the wheels on the track?
Uh... the flange..
NO. The wheel's profile. It's tapered.
Then I'd put a dixie cup on the table, give it a push, and it'd roll in a circle.
That's what railroad wheels do. One dixie cup on each end of a pencil, and two rails on the table- as long as the wheels are centererd... meaning, each rail rolled on each dixie cup at the SAME DIAMETER POINT, it'd go straight.
Start it off with it being higher on one side than the other, it'll FIND center... because the smaller diameter would not WANT to travel as far as the larger, the wheelset STEERS itself to center.
In a curve, the tracklayer must start the inside arc of a curve BEFORE the outside, so that it draws the inner wheel to smaller diameter of the wheel profile. This causes the axle... and every axle AFTER it... to 'steer' in an arc.
This is why there's stepped notches on that level- it's not to find level, it's to cause a correct arc in a curve.
There's a matching wheel profile gauge... so that the taper of the wheel matches the geometry that level creates.