The rover traveled along the line until it came to a crossing.
Then it sent a mail message to the next RCX.
The first time a merry go round on output A moved
then the RCX sent a mail message back to the rover.
The rover traveled along the line until it came to a crossing.
It sent a message to the next RCX.

The RCX with merry go rounds are programmed to wait for mail message.
The first time it receives a mail message the merry go round on motor A runs
then the RCX sends a mail message.
The second time it receives a mail message the merry go round on motor B runs
then the RCX sends a mail message.
The third time it receives a mail message the merry go round on motor C runs
then the RCX sends a mail message.

The rover passes the three RCX with Merry go rounds.
Each time a different merry go round runs.