Stage 1
The trains are stopped and the experimentors are timeing the Laser light.

ere the Laser light leaves Mi's train, goes up to Mo's train, hits the mirror and returns in exactly 2 seconds. We are assuming no extraneous factors such as gravity or complicated motions. Also there is no time of delay in mirror reflection. These are givens for this planet and experiment. Also assume the velocity of lihgt is 186,000 miles per second, and the trains are exactly 186,000 miles apart.

Above Diagram
Shows the trains stopped. the Laser light goes up to the top train in one second and returns in one second. Total time for the round trip is two seconds

Stage 2
This is the situation with Mi and Mo using the plasma light with the trains stopped. A shard of the plasma light travels up to the top train, hits the mirror and returns in a total round trip time of 2 seconds.