In order to unlock this achievement you need to finish the mission in 6 turns or less.
Use your fastest soldier to place the Transponders while protecting him/her with your other squad members.
You don't have to kill all the aliens on this mission. Simply place all the Transponders and activate the train.
ATTENTION!
This achievement seems to be bugged. If moving into the train activation area is your last action during the turn, it may not unlock, even if there are more than 3 turns left.
In this case just wait a turn after placing the last Transponder, and then activate the train on the next turn.