The functions read_M1() and read_M2() return the state of the
velocity meters within the specified block section BLOCK. While read_M1()
evaluates the velocity meter which is passed firstly by trains running in clockwise
direction, read_M2() evaluates the one passed secondly.
Please note that each velocity meter is only capable of detecting the crossing of a
train, not measuring its speed (see description of Velocity
Meters for details).
Please note also that not all block sections provide velocity meters. An attempt to
evaluate the velocity meters of a block section without will fail.
Evaluating a velocity meter not being crossed by a train results to
NOVALUE. If a velocity meter was crossed by a train, an arbitrary value
unequal to NOVALUE and unequal to FAILURE will be returned.
Any attempt to evaluate a velocity meter will reset it to NOVALUE.
If BLOCK specifies a block section not equipped with velocity meters,
FAILURE is returned.