At time, t=0, Alice will send Bob a message he will receive before she sends it.  The train is equipped with a hypothetical instantaneous signal delivery device.  Two photons at the center C are sent to the front F and rear R to verify that the message is received the instant it is sent.  Alice waits till the front reaches her to send the message that Bob had received a short while ago when the rear of the train passed him. The consequences of a prompt reply (from Bob to Alice) using a similar device is sufficient to convince most that superluminal communication is impossible. The small arrow in Alice's timeline marks the arrival time of this reply.  See also File:Causality_and_Superluminal_information_flow1.svg
Own work
Guy vandegrift and the authors of File:Emojione_1F468.svg, File:Emojione_1F469.svg, and  File:Icon_train.svg